Whether you not create a new owner group when you stop inheriting permissions in the subsite?
If you have not created a new owner group, you can not modify the allow access requests group.
The only way is to delete unique permission the stop inheriting permissions again.
To correct this same issues we had to find the group within SharePoint and open it to see the members. On this screen you need to go to "Settings - Group Settings", then changed the option "Who can view the membership of the group?" from "Group Members" to "Everyone".
Here is the code that worked.
Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue
#Function to Check if a User exists in AD