If you type the following command, nothing ever updates, but only in some worlds. Newly created ones usually do not have this issue.
/data get entity @s ForgeCaps."wings:flight"

This is something very useful for modpack creators, so this bug can break some modpacks' mechanics.

Also having this issue on 1.16.5
I've learned that /kill resets it, but if I leave the world and rejoin (Single player) the data freezes again.

Update: On my custom modpack, disabling the Endergetic Expansion mod fixes this issue.
Likely related to this: team-abnormals/endergetic#162