Cool that it is adjustable. I must have looked at a different one.
If the changing temp is slow, then I wouldn't worry about it. That's good that it's staying stable. I have a reliable heater that keeps my 10g tank at 78 at night, but with the weather getting warmer it gets up to 82 during the day. That's fairly common if the temp changes slowly with the ambient temperature of the house. And 80 isn't too warm for bettas- 82 is generally considered the warmest to keep them at on a continual basis.