I can't reproduce anything that you're reporting here.
I've done the following. Created a permission interface group with an ID of "test". Created a permission with an ID of "test", set the group ID to "test" and set the interface group to the one created previously "test".
No errors here, and there shouldn't be. None of this is invalid.
I've then created a simple template modification in a similar way:
Which saves correctly:
And displays correctly:
So, something else is going on here. We'd need more detailed reproduction steps to be able to go any further,
Thank you for looking into this Chris. I have noticed it works fine to have the Interface group ID and Permission group ID as the same name, that is until you disable the add-on and then re-enable the add-on, at this point the Template modification no longer works.