Mobs Can't Despawn

Thank you for your report!
We're actually already tracking this issue at MCPE-21856, so we will resolve and link this ticket as a duplicate.

If you would like to add a vote and any extra information to the main ticket it would be appreciated.

If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.

Note: Although MCPE-21856's description only talks about spawning, spawning and despawning are both part of the same balance algorithms and despawning has been discussed extensively in the comments.