This sounds like it could be a bug which you need to report direct to Apple. Here we are in peer support pages which Apple employees are not required to read.
If the watch and health app are not recording heart rate correctly you need to identify where the problem is appearing to make the most accurate report. Both Watch and iPhone had software updates recently, and your post doesn’t identify where you see the problem:
Heart rate is measured by the watch, and you can view today’s graph on the watch screen. It is also passed to your iPhone where you can view it in the Health app with history graph over a longer period, and you can also view the numeric data for each sample received from the watch. I can see several steps there where rate below 40 could get ”corrected”.
If you can get your pulse below 40 while awake it would be good to start by making an independent physical check, feeling with your fingers and a timer. You know that has to be correct, and you can then follow through on the watch display real time, the data sent to the Health app, and how the Health app plots it. When you know which step is dropping the ball you have a really useful bug report.
