Try the following - switch it off, switch on, slide to maximum (28°), wait a few seconds and slide to the minimum (18°) and see what is the reading on the power meter?I usually use my heater in my room, that must be around 25 m2 (or less) and in that case, the temperature keeps going up, even at 1200W (which seems to be the minimum power output). I used it once in my living room, that's probably around 45 m2 and, in that case, temperature didn't seem to go up once it changed to 1200W.
Could it be that the min. output is just too powerful for smaller rooms, making it impossible to maintain the room temperature?
I thought I read the heater also had a 800W output, but during the time I measured it, it didn't use less than 1200W. It also only completely turns off if I create an automation for it to turn off if it goes above a specific temperature (using the temperature and humidity sensor 2 and not the thermostat from the heater).
I would say the heater should have a power output under 1200W, like 800W or 600W, and that would probably solve the issue, I guess