The horse ain’t dead yet so I will keep beating it.
Britain, France, and the CP contest Belgium.
Britain, France, and the CP contest Lorraine also.
CP controls Ruhr and Alsace, France controls Picardy.
France moves into Lorraine from Belgium.
Now, if they attack and win, Britain CANNOT move their forces in to Lorraine. But, if it remains contested, Britain could move in. Unless Britain has enough transports for all the units in Belgium or they somehow have units in another adjacent territory that is contested with the CP (Which in the above scenario they don’t), they are trapped in Belgium.
By clearing out Lorraine of enemy units, France actually traps its ally in an adjacent territory.
However, if there are CP forces remaining in Lorraine to oppose the Allies, Britain has no problem moving in.
This really needs to go sooner rather than later, by allowing a power to move from a contested to an ally’s territory that had 1 or more of the moving power’s units in it at the start of the turn.