I don't really understand what you claim to be the Soviet propaganda. 400 years of the unified Kingdom of Georgia founded in 11th century?.. Mingrelia was one of the pieces of a single country torn apart by Arab, Persian, Turkic, and Mongolian conquests.
A lot of ethnicities are torn apart and stay that way. Ask Kurds. Or XXI century Russians for that matter.
I am just pointing out that Soviet Union has encouraged large ethnicities to coalesce, collect their shared history and culture (often inventing large portions in process) and build a nation state on top of that. This is not something you should take for granted, but that's what you've got under Soviet rule.
Not everybody got that privilege, of course. Ethnic Russians, but also Jews and Germans, did not get to have their own nation state in USSR. Technically, Jews had one but it was a joke[1]; Germans briefly had one[2]; Russians hoped to have one after WW2, with capital in Leningrad, but were derailed (Leningrad affair[3]). RSFSR was like a one huge Washington DC in that regard, with glistering Moscow and Leningrad but it was more like Puerto Rico as you descended into Nechernozem'ye[4]. No statehood.
https://en.m.wikipedia.org/wiki/Principality_of_Mingrelia
But the actual maps show a mosaic of principalities in place or Georgia.