Chickens won't die

I had a large chicken farm which was causing lag (~100 chickens). After I killed the chickens, they reappeared as if nothing happened when I reloaded the chunk (either by closing the game and starting it again or by flying away and returning).

I tried suffocating the chickens with blocks, burning them with lava, but they keep returning and the lag seems to be getting worse after each reload (loading a world takes minutes and major lag).

Villagers are affected, but in a different way: they keep escaping from an enclosure and appear in the exact same spots (after reloading the chunk), even though I led them to their enclosure before.

I'm on Xbox so I cannot share my world here. If anyone needs pictures, videos etc, please let me know!