This is a standard behaviour for SharePoint Libraries. In order to get to your Power App form, you need to 1) click on on the little information circle and 2) the click on the "Edit all" link:

Now you will see a "Hidden" Power App form:

Update
If you want to directy open the Power App form, you can do this:
- Create a new column. Make sure it always has data in it.
- Format this column with JSON formatting and use this code. Update the URLs accordingly.
{
"elmType": "a",
"txtContent": "",
"attributes": {
"target": "_blank",
"iconName": "OpenInNewWindow",
"class": "sp-field-quickAction",
"href": {
"operator": "+",
"operands": [
"https://TENANT.sharepoint.com/sites/ModernTeamSiteA/Shared%20Documents/Forms/EditForm.aspx?ID=","[$ID]",
"&Source=https://TENANT.sharepoint.com/sites/ModernTeamSiteA/Shared%20Documents",
"@currentField"
]
}
}
}
Result

