Tribal Governmental Plan Form 5500

We recently took over a tribal government plan and the prior TPA has been filing a Form 5500 for the plan since its inception. This plan truly qualifies as a tribal governmental plan and so it is not subject to the Form 5500 requirements. Should we file for the 2020 plan year and mark that it is the Final Form 5500 or not file and, when the IRS sends a letter requesting the filing, respond that the plan is not subject to the Form 5500 filing requirement?

I'm not sure if it is an error or a warning if you try to file a final 5500 with assets still in the plan...

Happens to us when taking over Non-Electing Church plans from TPA's who were filing 5500's.  We agree re not filing "final" 5500 because the plan still has assets.  We have a letter we help the plan sponsor send when contacted by the IRS about the "missing filing."  That approach seems to work fine.