I noticed this was being a bit glitchy, but couldn't figure out why.
Dreadnaught was destroyed, and seemed like I still had some units left -- air and ground.
So I tried to move them to be aded to another army (blue arrow).
SOME units were added.
Others weren't.
I don't know if this was because of the reinforcement logic (only accepting additions on the reinforcement list).
Either way, it was annoying, and it seems like the move should have overriden the reinforcement cmds.
E