1. Generally you can ignore that Garmin Vo2max number. It’s dependent on the maxHR you specify in their app and will be affected by hot weather or otherwise difficult conditions. If you do your trail runs (like that one sounds as if it is trail) as “trail” activity in Garmin, it will ignore it for Vo2max calculations.
2. Sometimes a chest strap HRM is influenced by static electricity and will cause spikes like you describe. Make sure you thoroughly wet the strap when applying, and/or use some electrode gel, otherwise you’ll occasionally see those spikes.