Why doesn't VO2 max change ?

Do you have a power meter on your bike? It is required to get VO2max for cycling.

Hard to say, going from >50 to 40 in just a couple months seems like a lot, unless you litteraly were doing nothing.  But... then how did it drop/update that much?  

2 weeks of training likely will not raise your fitness a whole lot unless you are doing a few super hard interval sessions in addition to your endurance rides.  Even then will take some time for body to adapt and improve.  I would give it time, did your weight also go up in those few months off?  that can cause vo2 max to drop as well.   make sure maximum heart rate setting and weight look correct in your profile.

Try to do a harder shorter ride and see if it updates (do 10-20 minute warmup... 20minutes nice and steady around your Threshold or sweetspot power (FTP 85-105%) with a 10minute cooldown.  usually and effort like that iwll update it.  

Are you getting a dot on the graph each week for vo2max?  Like that it is registering a confirmed 40... or is that just old data with no new dots on the app or garmin connect web ?

I have not used Power meter since I started using F945 to track my daily cycling and VO2 max gradually increased from 40-45-50 ... About 2 months ago VO2 max was 53. After a period of not cycling Outdoors due to the Covid pandemic (I ride my bike indoors via App like Rouvy, Zwitt) so VO2 max is only 40.

You can't get VO2 max values without a power meter. 

Did you track other activities besides biking on the days you got a new VO2 max? Walking also gives VO2 max values. 

I have never used power Meter. Only use HRM strap, Speed, cadence Sensor when cycling.

Very rarely turn on walking mode to track. So why does VO2 max increase ?

On some devices, the VO2max value could be updated for cycling activities even if you have not a power meter (its actually a bug, or at least its not a common feature). https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-6-series/228058/got-a-vo2-max-reading-from-cycling-but-don-t-have-a-power-meter-how

What does the 4 week VO2 max graph look like? 

Here is mine:

Green values are from running and walking. Blue values are from cycling. Look at your values. Which type do they have?

VO2 max at May 2021 (value 53)

VO2 max at Jul 2021 (value 51)

VO2 max at Sep 2021 (value 40)

Ok. Those values are not VO2 max from cycling. What other activities do you have in those days?

That's what I don't understand. I only cycle every day and use the garmin F945 to track the journey and parameters. Also don't run, don't swim. Walk only about 6000 steps/day, walk as usual