Dedmen stated this on Discord:

Also if you wonder why fog is bugged.
Fog works on rgb, thermal is grayscale.
Because reasons, the thermal rendering uses RGB rendering, and for precision reasons splits up the 0-1 thermal range, into RGB values each value doing 0-0.333
Fog is applied to these split up rgb values, instead of to the actual thermal temperature which makes everything all kinds of yuck
And because fog also works with fog color, and everything is built to apply colored fog, and thermals have no color....

Getting these changed would be the next step in thermal overhaul, it was too much work and too big of a change to do it with the first iteration

I hope this clears things up.