Running VO2max is calculated by a compatible garmin device for running activities with HR data. Some types of activities do not qualify, for example trail runs. Some devices can also estimate VO2max based on walking. Cycling VO2max requires cycling activities with HR and power data.
I think Mvrobel means that VO2max is currently not synced. This is what I experienced too. All values after a run are synced except VO2max. Started on my fenix5 after the 12.30 upgrade.