Mortar's Hold Fire doesn't work

After executing an attack move with a mortar it starts firing. And then, when I hit hold fire button it doesn't do anything. the mortar would keep firing.

The only way for me to actually stop the mortar from firing is to move it to another spot.

As far as I can tell, packing and redeploying a mortar (i.e. moving it) removes Hold Fire. The only way to get Hold Fire to stick is to give a move order or hit halt, let it set up, then activate Hold Fire.