Not seen that one, but have seen the one where artillery built on non-level ground (side of a hill, etc.) cannot fire at all points within its range. If the bombard ground is issued the artillery will move to shoot, the cooldown will begin, but no shells will be fired. It can be cancelled and the cool down immediately allows it to fire again. I think this is collision related but am unsure, definitely annoying.
Yeah it happens all the time. When positioning it, make sure to put it on flat ground.
That about decrewing: haven't really had such experience, can't say.