Can't load .dat and .nbt files (server list & saved hotbar)

I'm having a similar issue with my hotbar.nbt, it just won't load my hotbar or keep a saved hotbar config.

I'd suggest generalizing this report as this bit of code affects much more than the server list.
All .dat and .nbt files, at least, are affected by this.

scoreboards also affected

Maps and Refreshing server lists seems to be affected (Annoyingly, the latter persists in older versions and my server list has to be manually remade)

Maps are affected, please add

Maps were tracked separately in MC-266289, both issues are fixed for the next update.

Actually, that one only mentions raids and random sequences, not maps.
These 2 reports really are the same issue, but whatever.