VO2 max during the walking activity

Walking VO2max can be estimated by the watch, but it is considered unreliable compared to running VO2max, so the walking values are only used if no running values are available for the last 30 days.

So are you stating that the 255 automatically after 30 days of no running data will update VO2max report with that for walking despite the fact that after the walk it does not list the VO2 max.  Which walking date data does it use? Where did you find this information out?

If you look at your walking data in Runalyze, for example, you'll notice that it does contain updated VO2max ("VO2max from file" in Runalyze), even though that is not used to update the VO2max Garmin shows you, if you have a recent run with VO2max recorded.

I'm not sure I understand "Which walking date data does it use?". Garmin's VO2max is a constantly updated estimate, so VO2max for any activity is not from that activity alone, it's an updated estimate after that activity. 

For information, see "Reason 3: The Type of Activity Recorded Did Not Surpass a Previously Recorded Activity" from the link below

support.garmin.com/.../