Units that split from a stack bypass the zone of control penalty. (potentially the bug / feature)

The rest of the units can then also move away without being affected by zone of control.

Also, for some reason the units have 7 movement, even though the UI only lists 6. (slow speed)

Screenshot shows that moving the entire stack would apply a zone of control penalty (-3 movement)

Screenshot shows a heavy chariot split from army group (no movement lost)

Screenshot after the other 3 units moved to the heavy chariot and are now free to move away without any zone of control penalties.