I've used the dark master builder function in the past and it has worked well. Today, I decided to rebuild my dark library... but ran into a problem. With my ZWO 183MC Pro, I set the camera temperature at -10C. When I tried to build new darks, EKOS kept changing the value to 0C. I checked my defaults and it is set at -10C and I manually entered -10C in the camera tab. Regardless, it keeps changing it back to 0C when I start the build.
I just tested with my ASI294Pro. I was unable to reproduce your issue. This is a screenshot of what it did look like here:
Two things I could think of:
1. Are you sure you selected the Pro cam in the pulldown menu?
2. The first temperature value must be lower than the second one. If the first one is set to 0 you can not set the second to anything lower than 0.