Have done multiple 5km runs since those 3 first trainings, always on outdoor runs and always at top zone 4- low zone 5 so 80-90% of HR max. And it takes me way longer than 10 mins to finish 5km.VO2MAX Is calculated only in "Outdoor Running" activity. You have also to run 10 minutes with an heart rate > 75% of FCMAX. If in the stratos appears the data you can sync directly from the watch and then open Zepp app and do a swipe down to sync.
To give you a perspective since I trained casually, my only three readings put my VO2 max at 35 only. So not a seasoned runner.
It baffles me that I get all other metrics except VO2 max readings and race predictions.
Had some predictions during those first 3 runs that went all the way up to marathon times. Resetted the watch twice since then, lost VO2 max and race time predictions.
As i said in my initial post, all other Firstbeat calculations (training load, training effect and recovery) show up every time and correctly.