Very inaccurate (and inexplicably changing) VO2max compared with medical measurement

I have now had two Apple Watches. A Series 4 for over 4 years, and now a Series 8.

With the Series 4, my VO2max estimate was in the mid-30s for a couple years, and then it declined to the mid-20s over the course of about 6 months even though I hadn't changed anything about my exercise routine and didn't notice any change in fitness. Just before the apparent decline began, I had a full cardiopulmonary stress test and my VO2max was directly measured at 42.5.

When I got the Series 8 about a month ago, the estimate jumped a little, from 26 to 29. Then, earlier this week I had another cardiopulmonary stress test, and my VO2max was measured at 42.1. So, the medical measurements match and are dramatically different from the watch.

I understand that the VO2max estimate is simply an estimate and won't be accurate for everybody, but I'm wondering if there are any known factors that would produce something this far off. I know it's reasonably accurate for many people. I have a maximum heart rate that is significantly higher than would be predicted by my age (~10 to 12 bpm higher); this was confirmed by the stress test I just completed. Has anybody figured this out?