This example of Autumnal Woods Hills from Biomes Bundle. As you can see, before update it has yellow color (right side of screenshot), after update — default vanilla green.

Hey. thanks for reporting this. Do you see any exceptions related to packets in the logs when connecting by any chance? In-game do you see a red warning when connecting that a packet could not be received?

Yes, when stainding in that biome in console spam:

[20:22:30] [JM-task-4/WARN]: java.lang.NullPointerException

every 2-3 second 6-10 lines without stacktraces

I've not been able to reproduce this, can you delete OTG-Core and try again? And if that doesn't work, try a new world and see if that gives the same error?

Yes, new world with same seed works fine...
Is there any idea how to fix it in an already existing world?

Each chunk in affected region files (*.mca) contains wrong biome IDs.

I don't know, why biome ids in level.dat become different... I'll try fix it with NBT Explorer.

Hmm. level.dat generally does not affect.
I was able to fix it. Get from mca-file (from chunk) correct biome ID and put in to WorldConfig.ini.