I have fought a lot of times the Goose, and I know it sometimes fly away (during spring) so that it will come back when the moslings need protection. However, I recently had more times when the Goose flew away during the battle, it was constantly on the screen but while I kited it, it just gone. It's really annoying, because I can't do anything to prevent it and then I have to start the whole fight again, losing the invested time and weapons.
In the last times it happened as the Goose's egg was already cracked and the moslings there. If I remember well it never happened before the egg hatched, but I'm not entirely sure because it happened a few times earlier too.
Start a normal world, in spring fight a goose (possibly after the moslings are out of the egg). This bug happened rarely (maybe 10-15%?) but often enough to be annoying. I didn't notice anything common when it happened.