You need to add your managed property mapping as below:
<mso:ManagedPropertyMapping msdt:dt="string">ListID','ListItemID','CommentOWSMTXT','ContentTypeId','Link URL':'Path','Property 1':'Title','Property 2':'','Property 3':'','Property 4:'','Property 5':'','Property 6':'','Property 7':'','Property 8':'','Property 9':'','Property 10':'','FileExtension','SecondaryFileExtension'</mso:ManagedPropertyMapping>
In your item display template, use the code as below:
var desc = $htmlEncode(ctx.CurrentItem.CommentOWSMTXT);
var renderdesc = String.format('{0}', desc);
//render on the UI as below
<span title="Open Menu" class="NotificationDiv ms-ellipsis-a" rel="_#= viewPropUrl =#_" ref="_#= renderdesc =#_" path="_#= linkURL =#_" style="cursor: pointer; margin-top: 3px; float: right;">
When you do this, you don't need to specify it in the managed property dropdown.
After this, it started working at my end. The text 'Gebeurtenis' in the below screenshot is coming from my managed property:

My mappings from CSWP UI are as below. I have added only Path and Title values.

Explanation -
When adding additional property mappings to a display template, most of the time you will add them like this:
'property Name'{property display name}:'managed property'
'property Name':'managed property'
If you add properties mappings to the ManagedPropertyMapping tag like the two examples above, they will become available in the UI of the Content by Search Web Part.
There is another way of adding properties to your template and this is by just defining the managed property name like this:
'managed property'
Important: you do not define a property name and mappings, only the managed property. If you do it like this, you create a "private" property for your template. It cant be modified by users from the UI.
If you take a closer look at one of the OOTB CSWP display templates, you will find the mapping as below:
<mso:ManagedPropertyMapping msdt:dt="string">'Link URL'{Link URL}:'Path','Line 1'{Line 1}:'Title','Line 2'{Line 2}:'','FileExtension','SecondaryFileExtension'</mso:ManagedPropertyMapping>
Here, FileExtension is used as private property and cant be modified by user from UI. It is used internally for providing File Extension and related icons for a document.
Reference - Adding managed property mappings to your display templates which may not be altered
Display template code:
<html xmlns:mso="urn:schemas-microsoft-com:office:office" xmlns:msdt="uuid:C2F41010-65B3-11d1-A29F-00AA00C14882">
<head>
<title>List Table Item</title>
<!--[if gte mso 9]><xml>
<mso:CustomDocumentProperties>
<mso:TemplateHidden msdt:dt="string">0</mso:TemplateHidden>
<mso:ManagedPropertyMapping msdt:dt="string">ListID','ListItemID','CommentOWSMTXT','ContentTypeId','Link URL':'Path','Property 1':'Title','Property 2':'','Property 3':'','Property 4:'','Property 5':'','Property 6':'','Property 7':'','Property 8':'','Property 9':'','Property 10':'','FileExtension','SecondaryFileExtension'</mso:ManagedPropertyMapping>
<mso:MasterPageDescription msdt:dt="string">This is the item row template, use this template in combination with the List Table Control template.</mso:MasterPageDescription>
<mso:ContentTypeId msdt:dt="string">0x0101002039C03B61C64EC4A04F5361F385106603</mso:ContentTypeId>
<mso:TargetControlType msdt:dt="string">;#Content Web Parts;#</mso:TargetControlType>
<mso:HtmlDesignAssociated msdt:dt="string">1</mso:HtmlDesignAssociated>
<mso:HtmlDesignConversionSucceeded msdt:dt="string">True</mso:HtmlDesignConversionSucceeded>
<mso:HtmlDesignStatusAndPreview msdt:dt="string">https://mijnszr.sharepoint.com/_catalogs/masterpage/Display Templates/Content Web Parts/Item_List_Item.html, De conversie is voltooid.</mso:HtmlDesignStatusAndPreview>
</mso:CustomDocumentProperties>
</xml><![endif]-->
</head>
<body>
<div id="TwoLines">
<!--#_
// Give up your names for the properties in the following array
var propertyNames = {"Property 1":"Property 1 header value","Property 2":"Property 2 header value"};
var encodedId = $htmlEncode(ctx.ClientControl.get_nextUniqueId() + "_tableitems_");
var linkURL = $getItemValue(ctx, "Link URL");
linkURL.overrideValueRenderer($urlHtmlEncode);
var iconURL = Srch.ContentBySearch.getIconSourceFromItem(ctx.CurrentItem);
var listID = $getItemValue(ctx, 'ListID');
var listItemID = $getItemValue(ctx, 'ListItemID');
var ContentTypeId = $getItemValue(ctx, 'ContentTypeId');
var rowId = encodedId + "row";
var editCellId = rowId + "_cell";
var currentItemIdx = ctx.CurrentItemIdx + 1;
var alternating = currentItemIdx % 2 === 0 ? "ms-alternating" : "";
if (currentItemIdx === 1) {
_#-->
<thead>
<tr>
<!--#_
// Show the file icon
_#-->
<th class="ms-vh-icon ms-minWidthHeader">
<img width="16" height="16" border="0" src="/_layouts/15/images/icgen.gif">
</th>
<!--#_
// Show the properties that aren't empty
for(var i = 1; i <= 10; i++) {
var property = $getItemValue(ctx, String.format("Property {0}", i));
if(property.managedPropertyName !== String.format("Property {0}", i)) {
var propertyName = typeof propertyNames[property.propertyLookupName] === "undefined" ? property.managedPropertyName : propertyNames[property.propertyLookupName];
_#-->
<th class="ms-vh2">
_#= propertyName =#_
<span id="_#= property.managedPropertyName.replace(/ /g, '-') =#_">
<span style="" class="sortarrow ms-sortarrowup-iconouter" ></span>
<a id="_#= property.managedPropertyName =#_ASC" href="#" title="Ascending" onclick="$getClientControl(this).sortOrRank('_#= property.managedPropertyName =#_ASC');return false;">
<img alt="Ascending" src="/_layouts/15/images/sortaz.gif">
</a>
<a id="_#= property.managedPropertyName =#_DES" href="#" title="Descending" onclick="$getClientControl(this).sortOrRank('_#= property.managedPropertyName =#_DES');return false;">
<img alt="Descending" src="/_layouts/15/images/sortza.gif">
</a>
</span>
</th>
<!--#_
}
}
_#-->
</tr>
</thead>
<tbody>
<!--#_
}
_#-->
<tr id="_#= rowId =#_" class="_#= alternating =#_ ms-itmHoverEnabled ms-itmhover">
<td class="ms-cellstyle ms-vb-icon">
<!--#_ if(!iconURL.isEmpty) { _#-->
<img src="_#= iconURL =#_" />
<span id="_#= editCellId =#_"></span>
<!--#_
} else {
_#-->
<img width="16" height="16" border="0" src="/_layouts/15/images/icgen.gif">
<!--#_
}
_#-->
</td>
<!--#_
// Show the properties that aren't empty
for(var i = 1; i <= 10; i++) {
var property = $getItemValue(ctx, String.format("Property {0}", i));
if (property.managedPropertyName !== String.format("Property {0}", i)) {
_#-->
<td class="ms-cellstyle ms-vb2">
<!--#_
if(!property.isEmpty)
{
var value = i === 1 ? String.format("<a href='{0}' class='ms-listlink'>{1}</a>", linkURL, property) : property;
_#-->
_#= value =#_
<!--#_
if (i === 1 && !listID.isEmpty && !listItemID.isEmpty && !ContentTypeId.isEmpty) {
var desc = $htmlEncode(ctx.CurrentItem.CommentOWSMTXT);
var renderdesc = String.format('{0}', desc);
var viewPropUrl = String.format('/_layouts/15/listform.aspx?ListId={0}&ID={1}&ContentTypeID={2}&Source={3}', listID, listItemID, ContentTypeId, window.location.href.split('#')[0]);
_#-->
<span title="Open Menu" class="NotificationDiv ms-ellipsis-a" rel="_#= viewPropUrl =#_" ref="_#= renderdesc =#_" path="_#= linkURL =#_" style="cursor: pointer; margin-top: 3px; float: right;">
<img alt="Open Menu" src="/_layouts/15/images/spcommon.png" class="ms-ellipsis-icon">
</span>
<!--#_
}
}
_#-->
</td>
<!--#_
}
}
_#-->
</tr>
<!--#_
if (currentItemIdx === ctx.CurrentGroup.RowCount) {
_#-->
</tbody>
<!--#_
}
_#-->
</div>
</body>
</html>
<mso:ManagedPropertyMapping msdt:dt="string">ListID','ListItemID','ContentTypeId','CommentOWSMTXT','Link URL':'Path',....
mso:ManagedPropertyMapping msdt:dt="string">ListID','ListItemID','ContentTypeId','DESC CC':'CommentOWSMTXT',
then i use the followingvar DESCCC = $getItemValue(ctx, "DESC CC");
var renderdesc = String.format('{0}', DESCCC);
but did not work ..