VO2 max still broken, how to reset?

Looking at your runanalyze below, it does look weird. Obviously, some changes would occur if you do the right training

About the lab test, I am 100% sure you are correct; they would never be identical, albeit similar unless you are ill, fatigued or otherwise less performing.

But that’s basically my point: if you used your VO2max for planning etc., you would have to redo it every day if your scores change everyday.

A single score is one thing, but your overall fitness is another. If you (theoretically) did 14 lab tests in a row, you’d most likely get a median and average very close to each other. Your potential watch error aside, I actually think a “running average” (no pun intended) would work better than single-point scores, exactly due to the progression.

On another note, I am curious myself: how do you do the comparison below exactly? I know when I got my 955 vs. 945 vs. 935. 

Are you using a HR strap? This is very important. Garbage in Garbage out.

But overlall this makes sense, Runalyze does not take into consideration the heat, and usually drops VO2 more than garmin in the summer. Both make sense. Garmin tries to factor in the heat, runalyze just shows you your performance on a per run basis. The injury shows how garmin only slowly decreases your VO2 and Runalyze drops it immediately. Garmin flattens out right about when runalyze gets back to your pre-injury VO2.

The truth is your VO2 max would not decrease immediately when you are injured, unless it was a lung/cardiac injury. It could possibly decrease slowly if you went without cardio for an extended period of time, even then it would be slow and not what is shown above.

Same here, broken since last July, when Garmin changed VO2max algorithm (also VO2max stuck since then)

A status of maintaining can mean a number of things, but you're most likely stable/have plateaued and if you're improving, it's likely only marginally. Before I had COVID in November, my training status would remain Maintaining, but my vo2max estimate was steadily creeping up. To get Productive status by increasing Fitness (regardless of load), you'd need to start improving drastically and I've seen that only with people new to Garmin (no or very little data, so vo2max jumping all over the place).