Temperature coming from vents fluctuating between hot/ambient

Periodically I notice that my 2017 Prius Prime begins rapidly cycling the temperature between hot/ambient and it gets fairly annoying. For example, on a recent drive the outdoor temperature was around 50F. I had the temperature and blower set to Auto. The A/C was off and the temperature setpoint was 74F. For maybe an hour, it seemed to properly maintain this temperature. However, after a while it seems to get into a rhythm of full-heat and no-heat. The cycle period is maybe 10 seconds (I haven't timed it, so this is a guess). When everything is quiet I can also hear an actuator moving in conjunction with the change in temperature. This has happened several times and it seems to require some amount of time before it starts doing this (doesn't seem to happen immediately). I have only noticed it in heating mode. Has anyone else had this issue?