Cleric villager takes a stack of rotten flesh instead of indicated amount

On a multiplayer server which has no plugins or adjustments to villagers. A cleric always takes a full stack of rotten flesh for an emerald even though his trades shows that he should only takes 31 rotten flesh. When I supply him with 31 rotten flesh only and try to trade he would give me the emerald then take it back and gives back the rotten flesh. He only trades when I supply him a full stack of rotten flesh. This didn't happen with other trades with the same villagers tho. It also happened randomly