My friend backed up his Minecraft realm before 1.17, updated all the data packs, and tried to put it back on when realms updated. When it loaded all entities were gone, including pets and villagers.
He tried twice in a row and both times the same glitch happened. We decided it was best to wait for the 1.17.1 pre-release bug fixes, but as we didn't spot it on there and we haven't seen anyone else report the bug it seemed like a good idea to bring it up.

He suggests trying to upload any world on 1.17 to a realm to reproduce the glitch!
Thank you for your time!
(I can give a list of data packs if needed.)

Edit: Sorry!! I put it as java edition instead of java realms by mistake-