VO2 Max not properly calculating on Indoor bike with power meters

MileMuncher said:

I can see VO2 increasing a little bit every day with my outdoor rides, and decreasing every day with indoor rides, which are higher intensity and should have a greater impact on VO2.  

It is not unusual to see lower performance estimates (FTP or VO2 Max) with indoor biking than with outdoor biking, even in the case where power measurement differences are avoided like in your case.

The difference can be attributed to things like temperature (you use more respiration to deal with higher heat, less cooling effect) and less efficiency of pedaling because the bike doesn't move below you.

These factors can accentuate a bias in the power curve that you create if you tend to do intense intervals on the indoor bike (because they are easy to manage) and long/steady rides outdoors.

This is no big deal: the true VO2 Max is somewhere close to your estimates, and the trend over several weeks will reflect the benefit of your training, if any, on your actual VO2 Max.

For me it seems quite stable - I am at an age where its unlikely it will increase but it has held steady within +/- 1 over the last couple of years.

As I tend to ride outdoors when it is cool, my HR is usually low so if anything as my HR / power ratio looks good I tend to see positive performance condition which can lead to increases but my trend is essentially flat since 2021

I totally agree, however just to test things out I've doubled down and ridden with my forerunner 945 and EPIX 2 pro at the same time now both indoors and outdoors.  The forerunner is moving the needle on VO2 and the Epix is not for indoor cycling with power pedals.  Maybe the EPIX is more accurate, however seeing a consistent decline in VO2 every day during indoor rides on the EPIX seems like a bug. 

With the more recent models, Garmin has apparently introduced a dampening algorithm to the VO2 max updates.

it might be as simple as a moving average, it is not clearly published.

On my old Garmin 1030, my V02 Max can change by a point every time I ride (indoor/outdoor, hard/easy or other factors like heat, health, fatigue, etc). Not so on the Epix. Here the changes are way more progressive.

Having a VO2 max change every ride is actually not realistic and reflects more the limitations of the  estimation than real physiological changes.

So, IMO, we are better off with the dampened updates.

As noted above, all other things being equal, up and down changes when riding hard/easy can be a sign of inaccurate HR Max (this amplifies the error rate of the algorithm) and/or lack of balance in the historical training date (in particular in high/very high intensity segments).