VO2 max dropped

Do you have a power meter on your bike? Without it you will not get any cycling VO2 max (blue values). Then what you see is running and walking VO2 max (green values)

Post a screenshot of the VO2 max page with 4 week view.

Yes I do have a power meter on my ebike.

That looks good. The VO2 max is working.

I would start by comparing the activities and look at differences for the HR and power values between these new VO2 max values that are around 30 and the previous values that were around 45.

The activities are quite comparable. Max and average power is approx. 10% higher, max HR and average HR a bit higher compared to latest 29-30 VO2 activities. For me the VO2 value difference is not clear.

I do not know if this is normal but VO2 is measured soon after start from 6th min to 20th min of activity. Normal warm up takes some time and my personal max power output is generated mostly at the end of the hill climbs approx between 30th min to 60th min and not within 6th min. 

What you see is the performance condition: https://support.garmin.com/en-US/?faq=A28UA4k16v1qjjGuvSFgo8

"During the first 6-20 minutes of an activity the device will alert you to your current Performance Condition."

ok understood, thx. So the VO2 value is a result of the entire activity? Do not know why, but last bike session yesterday, VO2 value jumped to 38. The watch also proposes different trainings like base or sprint sessions during activity start. Do they have to do something with the VO2 level? Can I disable these messages?

1795997 said:

So the VO2 value is a result of the entire activity?

I don't think so. It will likely use all parts of the activity that meets the requirements for calculating a value: 10 minutes with over 70 % of max HR.