Okay, this has gotten way too opaque. As noted above all the German units starting in Breslau are identified as being inside the fort. The fort is attacked/assaulted - can't tell which order was in play. But the only unit shown to be defending in the battle graphic is the fortress!
That would fit your idea that the Breslau Division (and the Breslau Garrison this time 'round) were not actually attacked and so remain locked. But, since they're sitting in the same spot, why weren't they? And how the blue blazes does the system determine what gets attacked and what doesn't?
Finally something definitive. Ran it again and when the system decides to include potentially moveable units in the combat (in this case the Stadtgarrison and the Breslau Division) they are indeed freed for subsequent movement.