VO2max falling since 19.18 update

Some users including myself have seen an unexplained dip in VO2 max after an update. I saw it as 17.23 became the official version. My VO2 Max dipped by one full point, including on the detailed graph on the watch, then the VO2 max slowly went back up.

One explanation could be that a new firmware resets the VO2 max model. The next run estimates your VO2 Max during the first few minutes of a qualifying activity, just like on a new/reset watch. That initial Vo2 max is based on a formula linking pace and ventilation. Garmin describes it as your "minimal VO2 Max". Then the watch adjusts/learns your actual relationship between HR, HRV, on/off movement, ventilation and pace to improve your personalized estimates.