Closed MEP Filed Separate 5500s as an "open" MEP

Got a plan that has always been a closed MEP due to common ownership.  For some reason, it was thought by a previous provider that this MEP was open and needed the two employers to file separate 5500s going back to 2018.  My question is how do you amend a 5500 that should never have existed?  The only option I can really think of is to just amend last year's returns and show it as a transfer of assets to the "main" plan and as a final 5500.  

Agreed. And document everything.