Unexplained decrease in my VO2 Max

For the past 3 months my VO2 Max has been slowly decreasing from 42.7 to 42.0 for no apparent reason. Same exercise routine, sleep, diet, weight. Note that I had slowly increased from 39.0 in the previous 3 months, so have now lost about 20% of my recent improvements. I was expecting to slowly increase asymptotically to perhaps 43.0.
Is there anyone out there who pays close attention to VO2 Max, e.g., @Siva_Raj, who has seen something like this? I’ve been tracking for less than a year, so perhaps a seasonal thing, or stress-related due to the pandemic (I don’t consciously feel stressed though)?

Are you doing actual VO2max tests (daily? weekly?), or is it an estimate provided by some app?

Also tagging @Steve_Dean, because I know he’s given a talk on this.

I’m using Apple Watch, so it’s estimated VO2 max. Not sure if it is accurate, but it definitely appears to be precise and repeatable. At this point the decrease has slowed to about .01 per day, again, without changing my habits. I run 3 miles twice a week, and walk a couple miles on off days. The air quality index in my area (near Sacramento) has been fairly constant at around 30-40.

After a bit of research and digging through my data, it looks like the decrease in my VO2 max (estimated) is strongly (negatively) correlated to ambient temperature. Average temperature has changed from around 65F to about 95F during the time that my VO2 max has been decreasing. According to this study of runners and hyperthermia https://pubmed.ncbi.nlm.nih.gov/15150660/ that would lead to a few percentage points of reduction in VO2 max, almost perfectly matching my data. I’ll follow up in the fall to see if I get a “free” boost in VO2 max, and presumably an improved pace as the weather cools down.