Hi everyone,
So, I also have/had this same issue with my heater. I got it a few days ago and I noticed right away it wouldn't durn off once it reached the set temperature (18 ºC, in my case).
After reading this thread, I noticed that I had never clicked on the temperature in the homepage of the mi home app. After clicking on there, I could see an option called something like "configuration of information ..." or something similar. My app is set to portuguese ("Configurações de informaçõe..." is how it appears in PT), so not sure how it appears in the english one, but it's the option below the heater. Once I clicked on there, it read "Temperature" and I selected the heater as an input for the temperature reading (I suppose) and a tick appeared.
After that, I turned the heater on to 20 .ºC, then turned it back to 18 ºC and now it appears to be keeping steady at around 19 ºC (on the mi home app. On the temperature and humidity sensor 2, it got quite fast to 18ºC and is now climbing very slowly - much slower than before, I believe - and it just reached 19 ºC). Usually, by now, my room would be already at or near 20 ºC (while being set to 18 ºC)
I can't give any information about the power consumption, because even though I do have a smart plug, I'm still waiting to get my hub, so I can start using the smart plug. Once I get the hub, I'll try posting what I found.
Not sure if this is a fix or not or even if I was the only one that wasn't aware that you had to select the heater as the device for the temperature reading, but I thought I'd share this here anyway.