Do you use a power meter when cycling? If not, you'll not get any VO₂max value for any cycling activities.
From the article My Recorded Activity Did Not Update the VO2 Max Estimate | Garmin Customer Support
Cycling VO2 Max Criteria The Bike activity must be 20 minutes or longer without stopping. Heart rate data must be recorded (from either a built-in optical heart rate sensor or from a chest strap). Your heart rate must be elevated to at least 70% of your maximum heart rate for at least 20 minutes continuously. A power meter is required.
Cycling VO2 Max Criteria
