Technical detail from analysis
`frog_egg`'s blockID was renamed to `frog_spawn` in 1.18.20.21; however, there is no blockstate updater in the game which upgrades frog_egg from previous experimental versions, and the blockstate version also wasn't bumped to accommodate this change.

Boring detail
Steps to reproduce:
1) Create a world in 1.18.12 with "Wild Update" experiment enabled
2) Place some frog_egg blocks
3) Load the same world in 1.18.20.29
4) Notice the blocks have turned into update! blocks

I've attached sample worlds and screenshots to demonstrate the issue.