Epic jumps in altitude

Assuming you didn't manually recalibrate altitude at that point, is there any chance there was any liquid and/or dirt that was either in/covering the barometer hole and got cleared at that point or was introduced at that point? That's the most likely scenario to rule in/out first.

I've seen recalibration of 100 M when pausing activity. Did you pause at that time?

Maybe autocaliration, cannot it be? Do you have the auto calibrate altitude on?

Yes, I have auto calibrate. Thought it was only active before activity, not during pauses. Now I've turned it off.

Assuming you didn't manually recalibrate altitude at that point, is there any chance there was any liquid and/or dirt that was either in/covering the barometer hole and got cleared at that point or was introduced at that point? That's the most likely scenario to rule in/out first.

Clean as a whistle. Was happily running along a track, no change in conditions at that point.

I've seen recalibration of 100 M when pausing activity. Did you pause at that time?

This doesn't make sense. Why should it autocalibrate when I pause? (But no, no pausing, was happily running along.