Interesting! That uprising, was that the part of the clashes between germans and poles after the end of the war, or what? I'm embarassed to say that I haven't heard of it...
Take a look at this link. Not only does it explain about the uprising, it might explain the rumour about A7V involvement in Poland and whether there was another Heidi. Be amazed . . .
The Uprising seems to have been an attempt by the Poles to sieze Posnan from Germany and make it part of an independent Poland. The uprising was countered by the Freikorps, but the Poles clearly acquitted themselves well. They captured much Freikorps equipment, including, apparently, an A7V lookalike. Is this the "other" Heidi, the one with the turret, and is this how the tales of "A7Vs in Polish hands" began?