Palintonon/Throwers unpack themselves

Not sure if this is a new bug, as i think i remember it from 2011.

Nearly every quest, with all civs, when I command a packed-up siege unit to move to an empty piece of ground, it unpacks itself and starts attacking.
In fact, it even auto-unpacks when I haven't given a command.

I hit Pack button, but before it's finished transition animation, it unpacks and is attacking again.

Any idea why this is happening? (doesn't seem to happen to streamers that I watch)