I was not aware of the HRV metric until now.
Have checked my HRV in the Health app on my iPhone and it was 67ms yesterday.
Looking back to 2018 when I got my Apple Watch (series 3), the average has been fairly steady around 50ms over the past few years, when I look at the graph showing average annual results. However when I look at ‘all data’ and the daily values, some days have a low value of as low as 10 and other days have a high value of as high as 145.
According to https://blog.mymetabolicmeals.com/hrv-guide/ a healthy HRV is 100+. However I’ve no idea how accurate the HRV measurements are on Apple Watch.
All I can suggest is to see your doctor if you have any concern about your health.
And if your watch is still covered by AppleCare then request support from Apple if you’re concerned it may be faulty.