Apparently you're supposed to be able to limit the visual range or "temperature" like with a real thermal imaging device. I believe all FLIR devices have that functionality built in. There are commands but they are both limited and broken, so there is supposed to be a solution but I guess BI never checked if it worked. I made a feedback ticket for it that explains how and why it is broken and why it certainly needs to be fixed.
A BI dev has confirmed to me personally that you should be able to isolate and normalize a slice of the "temperature" range in Discord and provided further info on how it should work. I don't know how it took everybody so long to notice lol https://feedback.bistudio.com/T185536
With any luck, this will at least allow mission creators to get something usable out of the current system. It's a pretty crazy move to just thoroughly break a feature that worked good enough before in the last major update of the game and then just not fix it or roll it back or even do anything to improve it. I'm quite annoyed by this, honestly, especially since modders can't do anything about it. Just because this game is old and nearing the end of support and isn't actively being developed doesn't mean it's ok to make it worse or ignore it's problems.
