Vo2 max decreasing.

Hard to tell without seeing any data. Post a screenshot of the VO₂max report demonstrating the evolution, and links to some of your activities - an older one that triggered a high VO₂max value, and a recent one triggering the low value. 

Also, if you are increasing your base miles and not doing longer VO2 max specific runs it may how a decrease because you are not pushing yourself in high aerobic exercises for a long enough period. 

Just read another thread. VO2 not updating on Venu SQ2. This could be related. I have Venu sq2. Checked back and the same timing. Looks like a product issue. I'll ignore the numbers and trust my body. Cheers 

I have also found that after you have completed an activity (hit the stop button) dont hit save yet. Your heartrate will still be monitored and over at least the next couple minutes it will note your "recovery heartrate" which plays into the VO2 max estimation that it will give you. So, hit the stop button after the activity ends but dont take off your heart rate monitor if you wear a strap and wait at least 2 minutes before you save and close out. See if this gives you more VO2 data points. 

K-Napp said:

"recovery heartrate" which plays into the VO2 max estimation that it will give yo

I do not think it is true, and the whitepaper on VO₂ from Firstbeat Analytics (the provider of the VO₂max algorithm) does not mention recovery HR at all. It would also make little sense, since VO₂max is the maximum rate of oxygene consumption, which happens when the body works near the Lactate Threshold, not during the rest or during the cooldown. Although the Recovery HR is indeed a valuable metrics indicating your fitness, it has no direct relation to the VO₂max. 

I agree it may not be a direct correlation but it does give more data to show that fitness has increased because if you are improving in fitness your recovery heartrate should become more stable/lower faster that before. In regards to VO2 again, the run activity must be at 70% max heart for a continuous 10-15 min duration or 20+ min cycling to get a VO2 data point. Both again are showing a data point for that particular activity which may vary a little but if fitness is increasing the recovery heartrate should become lower/more-stable and the VO2 data should go higher over time.