It is problem with the mods. In particular the way they define features in their biome configurations.
This is an explanation of the problem.
https://misode.github.io/guides/feature-order-cycle/
You can try that Cyanide mod mentioned on that link to see if it gives you more definitive information.
Where the mod author you spoke to is correct is that minecraft has remembered this information in its world save.
To fix it, you would probably need to modify your save file. I have never tried to do that, so I can't tell you how to do it.
But the initial problem is with the mod.
I am surprised the mod author won't help you fix it if it is their mod that has broken your save file?
You can try asking one of the other mod developers I mentioned above to see if they are more helpful.
I don't think deleting chunks would fix the problem. The issue is not directly with the chunk, but with the biome the chunk uses.
It is the biome configuration you need to fix.