VO2 max with Apple Watch Ultra and Strava

Last January I shifted from Apple Watch 6 to Apple Watch Ultra. At the same time, I shifted from using Nike Run Club to Strava to monitor my runs. As well, I started running much more regularly on a treadmill than outdoor. After this, the VO2 max recorded by Apple Fitness has been tremendously going down over the last two months, despite the fact that I increased my volume of running by roughly 30%. I checked with Apple Support but they could not give me a definitive answer. I'm wondering whether this is due to lack of syncing between Apple Watch Ultra and Strava. Anybody has been experiencing the same? Can anybody advise? Thanks