May relate to MC-159560.
The bug
Color map textures will always pick from the bottom-left half of the texture file, including the central falling diagonal. This means that the entire top-right half of the texture goes completely unused by the game. Despite this, it is still colored in, particularly in the case of the foliage map.
How to fix
Remove all of the pixels from the top-right half of each colormap texture. I've included a fixed copy each for the grass, foliage and water (see MC-239129) color maps.
Linked issues
is duplicated by 1
relates to 2
Attachments
Comments 4

I don't have a code analysis on me but according to the wiki:
"The clamp limits the range of the temperature and downfall to 0.0–1.0. The clamped downfall value is then multiplied by the 0.0–1.0 adjusted temperature value, bringing its value to be inside the lower left triangle."
If anyone can back this up that would be great.
Provided there are still no steps to reproduce, and no code analysis, I am setting this to plausible for Mojang to review.

Thank you for your report!
After consideration, the issue is being closed as Won't Fix.
Please note that this is not the same as Working as Intended, as this bug report correctly describes behavior in the game that might not be the intended or desirable behavior, but it will not be fixed right now. Sometimes, this is because the issue reported is minor and/or impossible to change without large architectural changes to the code base.
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Do you have a reliable way to verify that the top half is completely unused? I believe the report, but to be through enough it would be helpful how to confirm this behavior.