I’ve done that on metal pipes to get more heat into them (more length of heat tape per ft) . not pvc since it does not conduct very well and the top is full of air which is an insulator. All the heat tape I’ve seen uses resistive wire that self regulates. It’s a safety feature. Only problem is that the hotter it gets the more it throttles back. Usually not a problem but in your case the majority of the tape is not at the bottom where it is needed and insulated to boot
Your setup does not sound horrible. Another thought is that maybe the pipe is pulling cold outside air. Like if your home is negatively pressurized.
