When I start up a reactor in a new game (without having researched advanced reactor, while using a working reactor blueprint from a previous playthrough) it goes into negative heat numbers and then explodes. I didn't notice this behavior with the previous playthrough.

Negative heat numbers mean the temperature is over 1000, otherwise it had only displayed 1.0k
Quality reactors can go over 1k without exploding, so it's useful to know the exact value.
Before, the temperature was capped at 1k° where the reactor would instantly explode
The combinator output doesn't use this negative number workaround

If you use control rods and have the "control rods temperature multiplier setting" changed (>0), it's possible that the reactor applies a temperature offset and immediately explodes when you start it up, but you said you don't have that researched so I guess there's something else wrong with your setup

yeah I don't know I think i might have to research the control rods - I tried it in a test setup and it worked fine, started warming up, and then in my vanilla startup base it just explodes every time - like ticks after it starts, as if it says - 15, -32, 650 or something, and then it just blows up. It's a good sight the white light and gameover screen

Idk but im gonna continue experimenting with it thanks

Even when the attached heat pipes are cold?

Maybe some mod messed with the heat capacity of the vanilla heat interface, which this mod copies.
The new version should fix that

I just reset the mod by uninstalling and reinstalling and it seems to work - it was weird because I would go to other playthroughs and it would work fine