When upgrading a world with custom biomes, all already-existing chunks with a custom biome will have their biome set to minecraft:plains, even with simple biome and dimension definitions that should work on both 1.17 and 1.18.

Steps to reproduce

• Download the map file Biome Bug World. This map was opened in 1.17.1 and the chunks surrounding spawn were loaded.
• Open the map in the latest version (currently 1.18 Pre-release 4)
• Press F3 to see that the biome is minecraft:plains. Should be biomes:custom
• Teleport out /tp @s ~1000 ~ ~ to see that newly loaded chunks will have the correct biome.