villager randomly multiplying

The villagers on my iron farm has only 20 villagers on it.. Two of the holding cell has 8 villagers
and one containing 4 . But the next time I checked it , each holding cell has some extra villager (roughly 20 villagers) with a different proffesion I assigned them to be(fletchers) . PLEASE FIX THIS BUG!! this bug occured on my realm by the way