After ANOTHER instance of this same issue again today, I can confirm more closely what is happening: when the issue occurs (after a GNSS-based activity), the calibration feature locks out until the watch is restarted, even if that’s days later. What this means is the altimeter drifts from reality as time goes on, and manually calibrating it won’t stick in the readings more than a few seconds, when the old value overrides. It does not appear linked to rinsing the watch (aka quick pressure changes from water), as it happened without that occuring.