It appears you have two things causing the issue. The first is that you cannot remove the artillery which are locked in the region. The second is the game is treating the region as if it has harsh weather.
Because army handling has the same problem minor allies have in WON in that you cannot remove a unit from a stack containing a static unit, some element of the game engine is contributing to this problem.
Probably the only solution is to back up a turn, and make sure not to merge your army with the static artillery.