The old PalettedContainer deserialization process called Palette#read(ListTag) to initialise the palette object's ids. This read method did not eliminate duplicates.
The new PalettedContainer deserialization process now calls Palette#idFor for each palette entry - this of course will eliminate duplicates.
The elimination of duplicates will cause the chunk to error upon loading, or at worst will destroy the chunk's data due to the palette ids possibly being shifted down (due to an elimination of an entry) by 1 or more. It could also have zero effect.
The vast majority of chunks do not have invalid palette data, but some do (ancient chunks? stupid mods?). And these chunks cannot be loaded in the snapshot now, whereas on 1.17.1 they load and operate completely fine.
Exception of me attempting to load an old world (Can't post the world): https://gist.github.com/Spottedleaf/2419b8bba4aef3e1f982737f5795e42e
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duplicates 1
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I created a fresh 1.7.10 world and loaded it (and attached it here), almost every chunk failed to load
Perfect, thank you! One more question, do you think this is the cause of MC-237925?
EDIT: actually, no need. I made a 1.12 world and got the same palette-related error in the log. I'll go ahead and link these two then.
Dupe of MC-237925
If it's not possible to upload that particular world, could you still please provide an alternative one? Would be very helpful for confirming this