Named polar bears despawning on an MCPE Realm

My friend took and kept a polar bear inside a secure place in our base and named it Finn. But a few hours later, Finn disappeared when we re-logged back into the Realm. Nobody could have killed it because it was a secure place and only us knew about it. Is this normal or a bug? Does this happen to other animals? Does this only occur in Bedrock/PE? Please let me know.