This is EXACTLY the confusion I was referring to. Garmin telling you your VO2 Max is improving is them looking at your heart rate.
TR prescribing a VO2 Max workout is looking at power output. I don’t mean to imply TR aren’t aware of (or don’t use) the HR data, but the last time I remember discussing this topic, @Caro.Gomez-Villafane gave this answer (click to see her whole reply for the relevant part):
BTW: That’s a pretty interesting (and short) thread, if you have time to read the whole thing.