I've found a problem with the 'USSR occupies eastern Poland' event, but also, luckily, a fix I think.
I noticed that when Axis units are occupying one or more of these 'eastern Poland' tiles at the time Poland surrenders, these Axis units are often (perhaps always) not moved out of the area when it is placed under Soviet control. They therefore get 'trapped' in eastern Poland. I've seen an armour unit trapped in Vilna, for example, never moving (AI controlled the Axis in that game).
However, if you edit the surrender_1 script, and remove the lines that assign the eastern Poland tiles to Axis (immediately before they are again assigned to the USSR, so these lines are not needed anyway), the problem seems to be resolved.
Perhaps there is a bug in the game engine, whereby units are not moved out of reassigned territory if, immediately prior to the reassignment, that territory was owned by the same side those units belong to? Maybe.