In the autumn my heater is likes that and I usually add a second just to keep the heat up. I had one in my bettas tank i think it was superfish and I had to turn it up high just to get it to 27 so yeah I think some heaters are like that. Maybe change it for a new one if you're concerned.
70 gallon elephant nose tank, Gonzo the elephant nose and inverts.
30gallon borara merah and brigittae green, red longfin bristlenose, shrimp, ninja, taiwan,bamboo, and snails.