I get this quite often. At some point - mostly after being attacked by enemies I think - the catapult will hover in the air.
If you go to where the catapult should be standing you can still activate it and you hit an invisible wall, as if it’s still there. If you activate it suddenly you get carried up into the air to where the catapult is on my screenshot, looking down on the map. Really weird :)
Makes the catapult unusable too.