REPRO:

1. In mission editor, place any soldier as the player. Place another nearby subordinate unit, set to playable.
2. Preview mission.
3. Team switch to unit 2.
4. Modify your stance (Ctrl+W or Ctrl+S) to any non standard stance
5. Team switch to unit 1.
6. Order unit 2 to move to some destination. Observe results.
7. Order unit 2 to switch to another stance (eg, stand up, crouch, prone), and move to some destination. Observe results. Note they can generally move if the AI wants to be in the general stance they are stuck in.
8. Repeat test, leaving unit 2 in other modified stance.

Alternative to steps 3 through 5 is to use script commands, for example:
(units (group player) select 1) playMove "AadjPknlMstpSrasWrflDdown"

If left in prone low stance, AI is unable to move.
If left prone left/right or prone upright, AI is able to move slowly, but only if they want to be prone.
If left in crouch low, AI will be able to move unless they want to be crouched. If they want to be prone, AI will be unable to stop moving.
etc, etc