Many people reported this, and indeed there is a bug with the generator. It should use old temperature/humidity calculation if a world was created before 2.1. But it does not – it uses the new one. As a result old worlds are a garbled mix of old and new temperature/humidity. This is now fixed and I should be releasing it soon.
If you still keep a backup of your <2.1 world, opening it in the upcoming bugfix release should work all fine. If you “baked” the errors by making modifications to the affected chunks, these chunks (and only these) will unfortunately stay garbled. There’s no easy solution to that.
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Source: Terrain from 2.0 has cold/hot patches after opening in 2.1