Axolotls from buckets despawned

I had two nametagged axolotls that I got from a coral reef. Acidic Strawberry an adult leucistic  axolotl and Cypher, an adult cyan axolotl. I logged out of the game with both axolotls in a pen I had made them, along with a baby axolotl I bred, that one didn’t have a name. The pen was closed, there was no way they could have escaped. The next time I logged in, all three had despawned. I had logged out in the area of the axolotls, so I hadn’t traveled that far from them. I was very confused and searched around. No axolotls.