Mobs disappeared on update to 1.17

Upgraded my server from 1.16.5 to 1.17. Numerous mobs have disappeared, including, but not limited to: horses, caged zombie pigmen, sheep, cows, bees, tropical fish.

(screenshot: pen on left previously contained ~30 sheep, most white, some dyed red, yellow, blue. pen on right previously contained ~20 cows.)

Players also reporting problems with redstone mechanisms not working, gravel not falling, mined blocks disappearing before they can be added to inventory.