It use to be on Garmin devices from circa 2010 with barometers, that they'd only show the uncorrected elevation given by the barometric sensor. If an elevation correction waypoint was set, and you happen to be near that waypoint at the start of your ride, they'd adjust everything based on that elevation. However it was a one time correction and only if found during the first few minutes of the ride.

Don't know what they do today. I don't use them for actual elevation. They do still give as decent as any estimates of elevation gain/loss if one isn't a bean counter and can live with the inaccuracies that occur on some rides.

I'd just ignore the elevation data from a ride you know is suspect. If you've done other rides of the same, then you'll have an idea what is more correct.