MinerJohn

This is happening on a friend's Realms server.  It has broken a previously-working ice farm

This has been happening to me too on a friend's Realm since 1.16 came out.  The server often reverts after a period of nobody playing to an earlier state.  Can't be sure about the intervals or how far back it rewinds but there is definitely an issue.  It has happened to other players on the same world

Seems several others are having this; possible duplicate reports at MC-100285 (mine own) and MC-100283

I can replicate this - rolled a backup of my singleplayer world back to pre 1 and zombies started spawning immediately.