Old nether chunks become overworld chunks

All of a sudden, when updating to the lastest version, which is 1.21.40, all my old nether chunks (which were created prior to 1.16) have turned into overworld chunks. This means that instead of being a nether biome, it is now an overworld biome, spawning hostile mobs like zombies and creepers instead of ghasts and magma cubes.
Steps to reproduce:
-Open an old world created prior to 1.16 with old nether chunks
Expected results:
-Old nether chunks shouldn't be converted into overworld chunks