I had a little bit on an adventure tonight trying to replicate the problem you had. I think I first noticed it when I turned on "stay off on boot" in the heater's equipment settings, may not be the item that caused the error though. At any rate, it started doing exactly what you described where the heater would turn on in the equipment control but was not being controlled with "Temperature". The only error I was getting was:
{"error":"not found - API documentation can be found at
https://io.adafruit.com/api/docs"}
The work around for me was to create a new temp control - the original one I've had from the V3 release is still not working despite having the same exact settings.
Should mention that I upgraded from v3.3 to the last v4 release about 3 weeks ago, and to v5.0 a few days ago.