Lugie it was the MP version but that doesn't matter. We all were looking in the wrong place. The problem is not the mortar team or the weapon but its the vehicle transporting the mortar. If you garrison the Russian mortar to a German Sd.Kfz 250 it will work normally but if you garrison it to the M5 half-track the problem will appear.
I think that this is might be an unsolvable problem. If you garrison mortar into an Sd.Kfz 250 the mortar WILL set up but when it is in M5 the mortar WILL NOT set up (and its not a problem of how many models are in the squad).
So i tried testing with the 250, and just like you said, it works just fine. To eliminate all possible variables from suspicion, i then created a clone of the working 250, where the ONLY thing i changed was the entity blueprint (from the 250 to the M5). That was indeed enough to break it again. Like i mentioned, i already tried adding the syncpoint line to the M5 halftrack, so unless i'm missing something, does that mean i'm fucked?