Are you using power? Are you recording your TR rides with your Garmin or are you letting TR sync with Garmin Connect? You need power on the bike for the VO2Max to work correctly (I’m pretty sure…) and rides coming from outside sources that don’t take advantage of Garmin’s Physio True Up function won’t be taken into consideration for the metrics that drive VO2Max or the recovery features.
It works fairly well for me on my 935. The actual score may be slightly inflated, but it still shows good trends and the recovery advisor is fairly accurate as well.