Productive Training Status with Decreasing VO2max, is it possible?

It's certainly possible. In fact, it happens all the time. Your training load is trying to get you into a goldilocks zone, neither too much training, nor too little. Being in that zone doesn't necessarily guarantee results, though if you stay mostly in the green, then you're probably doing pretty well.

VO2Max is measured by assessing how aerobically stressful it is for you to run at different speeds. It can go up and down to some extent based on fatigue, weather conditions, or even terrain.

Also, keep in mind a few things about VO2Max: Training doesn't typically move your VO2Max very much, or very quickly. You seem to be reasonably fit already, so I wouldn't expect VO2Max to change much at all. And a single point is really just measurement noise. It isn't that precise. It will typically bounce around within a very narrow range, occasionally shooting up or down because of an outlier workout or two, before settling back to where it was before. Look for long term trends, where it stabilizes at a higher or lower zone. Finally, remember that there is much more to fitness than VO2Max.

Thank you, JRMile :)

No Fred, I don't forget that :) What I feel in an individual run does not mean I get fitter, or less fit :)
I just want to understand more about this training status and training effect, to maximize the fun playing with this "toy".

My previous watch is FR630. My early VO2Max estimate was 45-46. Now it's hovering at 52-53, reaching 54 in some occasion. Along this increasing VO2Max, I improve my time in 5K and 10K race significantly. Also, I can run longer at much higher average speed. Still, I cannot reach the race predictor on the watch. There are threads about this race predictor too :)

No Fred, I don't forget that :) What I feel in an individual run does not mean I get fitter, or less fit :)
I just want to understand more about this training status and training effect, to maximize the fun playing with this "toy".

My previous watch is FR630. My early VO2Max estimate was 45-46. Now it's hovering at 52-53, reaching 54 in some occasion. Along this increasing VO2Max, I improve my time in 5K and 10K race significantly. Also, I can run longer at much higher average speed. Still, I cannot reach the race predictor on the watch. There are threads about this race predictor too :)

Join the club... I realise there's a 5% margin of error on the Firstbeat prediction, but for many people it always seems to be off in the same direction (too optimistic) and it could not be explained away by lack of endurance etc. When I tested 61 (metabolic cart) the watch gave me 64 on the same day. Within the margin of error, but enough off to make predicted race times useless.

Hi Fred,
How to calculate the "within error" of the predicted race time.

For example, my predicted race time for 5K: 20 minute. The average pace would be 4min/km, or speed 15km/h. So the "within error" would be 95-105% of the average speed 15km/h?
thanks.