In my testing with those reproduction steps, PAM's debug output says the module is running and adding the test group to the list of groups. So I'm guessing something lightdm is doing resets the group list at some point past that.
In my testing with those reproduction steps, PAM's debug output says the module is running and adding the test group to the list of groups. So I'm guessing something lightdm is doing resets the group list at some point past that.