After upgrading this Minecraft world from 1.21.11 to 26.1 it completely corrupted all the border regions ("region" being a set of 32×32 chunks, with a chunk being 16×16 blocks), very weird. All previously-generated chunks and new regions are fine, but any partially-filled region (at the borders of the area I had explored in 1.21.11) that I moved into had all the ungenerated chunks filled completely with air, so there were just massive voids around the entire area I'd previously explored. Could only get it fixed by going in and deleting all the empty chunks and then they all regenerated fine