I think this is a bug that should be fixed. Currently any unit that is further than 3 hexes away from enemy and can reach a hex connected to ANY NON-ISOLATED depot can disband.
Someone defended that the current situation is OK since you'll lose the counter when you disband, it won't return. Well, I don't mind the 7th Tank Division counter in the below picture disappearing, if I get to keep the 72 guns and 415 tanks when I disband it. Same applies to HQ support units, too easy to just disband or move them.
A hopefully easy way to correct this would be to add a condition for off rail MP since that is a number still being tracked.
This could be anything from 10 to maybe 30MP? I'd set it to around 20MP myself.
Actually moving SUs from HQs should be disallowed if the unit is too far from rail network (off rail MP!).
In the following situation, I think that most of the units in the almost closed pocket shouldn't be able to disband. If the off rail condition would be added, only the units marked in yellow and red might be able to escape if the Germans wouldn't be guarding the pocket bottleneck. If the pocket was more loose (2 hex gap on the bottleneck), even more units might be able to escape by disbanding, depending on the off-rail MP.
