Interesting one here. So it seems like the events are returning to the tile the chicken event was on when it unspawned upon being respawned as the other berry bush event.
This is happening with my spawner, for a reason I'll soon be looking into, but not with other spawners because this spawner recycles events. So when you unspawn a chicken that event goes into a sort of 'unspawned events pool' which the spawner looks to before spawning another event. if there are any unspawned 'null' events available it will grab one of those and transform it into a new event instead of creating another event on the map. Whereas galv spawner will just spawn a new event every single time you spawn an event.
The problem seems to be that the event wants to return to the position it was in when it was last unspawned.
spawn chicken
unspawn chicken
spawn berry bush
berry bush wants to move to where chicken previously was.
I'll take a look into this and hopefully have a fix soon.
Edit:
I've located the problem within altimit movement code, deciding on the best fix.