This is the internal representation, the three types of values are separeted by ;;
:
AudienceIds(Guids);;DistributionListsNames;;SharePointGroups
So when you have ;;;;GroupName
it is just showing that there is nothing from the other types.
You can check more about that here: https://docs.microsoft.com/en-us/previous-versions/office/developer/sharepoint-2010/ee906649(v%3Doffice.14)#target-a-web-part-to-a-specific-audience-programmatically
Relevant part:
You can assign three kinds of audiences to the AuthorizationFilter
property. To target a Web Part to a global audience programmatically,
you can retrieve the GUID for the audience you want by using a
AudienceManager object and then assigning that GUID to the
AuthorizationFilter property of the Web Part. You can also assign the
Lightweight Directory Access Protocol (LDAP) distinguished name of a
distribution list or the name value of a SharePoint group to this
property.
The SharePoint Server 2010 framework requires that these three kinds
of values be separated by a pair of semicolons (";;"). Multiple values
for global audiences and SharePoint groups are delimited by commas,
and multiple values for distribution lists are delimited by newline
characters ("\n"). The following example builds a string that would be
valid for assignment to the AuthorizationFilter property.
string[] audienceIDs = new string[] {"GUID", "GUID"};
string[] distributionLists = new string[]
{"LDAP Distinguished Distribution List Name", "LDAP Distinguished Distribution List Name"};
string[] sharePointGroups = new string[] {"SharePoint Group Name", "SharePoint Group Name"};
string result = string.Format("{0};;{1};;{2}",
string.Join(",", audienceIDs),
string.Join("\n", distributionLists),
string.Join(",", sharePointGroups));