After playing around with the Inkbird a bit more I was able to figure out why it wasn’t turning the heater on as it should, and it was totally my fault. In the settings, there is an option to set how cold it has to get before it turns on the heater. This setting was at 2 degrees. So when I had the controller set at 25 degrees, it wouldn’t turn on the heater until it was down to 23 degrees. After changing it to 0.1 degrees, the heater was coming on as expected.
I haven’t added this back to my system yet, but I will in the next few days. I also discovered that my bigger heater wasn’t turning off so the thermostat on that must have been broken as well, so a combo of the two compounded the problem. Now I have a working heater and I’ll install the Inkbird again tomorrow to keep an eye on things.