Superflat Chunk Deletion?

So I opened my superflat world after the 1.19.0 release yesterday and found that all builds that I had made beneath y=0 had been either completely or paritally deleted. I tried loading several different backups from before the update and none were able to restore the missing (what I assume) chunks.

I've added photos of two instances where we saw missing builds that were there previously.

One completely deleted, the other partially.