Axolotls de-spawn after being released from bucket or being name tagged

First I had two axolotls and I put name tags on them and released them from a bucket to keep them from de-spawning, within a few days they had de-spawned.  I got four more Axolotls and put them in the same spot as the other two but this I didn’t name tag them, I only released them from the bucket and again, they de-spawned. The axolotl types that    de-pawned were: adult gold axolotl, baby brown axolotl, adult cyan axolotl, and adult leucistic axolotl