In my F3, I could enable native data fields for ambient pressure and barometric pressure in a custom data screen in an activity. I can't find any such option on the F5X+.
Have they pulled it, or am I just looking in the wrong place?
With CIQ, both ambient and MSL is available based on what the FW calculates, so that is what you should see.
Activity.Info.ambientPressure and Activity.Info.meanSeaLevelPressure in CIQ terms!
For discrepancies, are you sure nothing changed? That can include just opening the fridge or a fan coming on. Or a strong wind. And temperate comes into play, so if the watch warmed a bit.
I think its a known issue and Garmin will fix/add it via fw update.