Hi, That is a strange one, but two things come to mind. It’s unlikely in such a new heater, but the dip tube may have failed or be in the wrong place. Second is water quality. If it were really hard, and you used a lot of it, sediment might have built up enough to slow the thermostats response. If neither of these is a problem, I’d ask for he manufacturer about testing the control and possible replacement.
Yours, Larry