In certain cases, it seems that the Dryad's Nimbus will get stuck on its home pad in the Altar of Athame Incursion. It will attack normally, but between rounds it sometimes ends up at his pad and will not move again for rest of game. The Tree was still fine, and I built 2 of them to test this and they both froze after the first wave. They were both attacking assassins at the end. Next wave they were not moving so I sold them after giving them 30 seconds of battle time to come back, and then built new ones in same place that worked rest of game.
Not sure on how to recreate, but seems to only happen in this incursion for me.