[Bug] [Priority:High] Loading worlds from older versions of minecraft (1.14.0 or earlier) causes massive corruption

When I open certain worlds in minecraft 1.16.0 and 1.16.1, they become corrupted and unplayable (especially in survival) because certain chunks of land go completely missing. I've attached screenshots showing the bug as well as 2 (uncorrupted zip format) worlds that corrupt themselves when opening in minecraft 1.16. Please fix this ASAP. This is reproduceable across android and ios. All the screenshots were taken of world 1. This bug occurs in both world types (infinite/old).