Weird assault result

[Mowgli]

Maybe I'm a bit rusted in regards to the rules, but I had something odd happening to me in my last Scheldt game: 

Germans assaulted my troops in hex 21.9 successfully (i.e. all defenders disrupted, at least one attacker not disrupted) and instead of the survivors retreating to hex 22.8 (no obstacle in between, friendly units present), what happened was that my units took the 50% extra losses as if they had no path of retreat. Really strange. At least some units could have retreated to hex 22.8 (not all due to stacking constraints). 

Any ideas? Am I missing something? 

(Screenshot attached; I can send a savegame)

I had this happen recently. I believe that the entire stack tries to fit into the retreat hex, and if it can't then it is treated as a "no retreat" situation.

Yes that would explain the situation. 
But I'm pretty sure that I've seen "partial" retreats in many previous games?