Running activity - wrong elevation tracking

Hi guys

I think I've had a problem with my elevation gain during my latest running activity.

I'm based at approximately 470m.a.s.l. So based on the elevation plot my watch apparently was calibrated correctly when I started running. But it kept declining for the first 8k to around -200m.a.s.l. Up to that point the elevation was actually probably between +/- 10m. Then it somehow got a hang of it and jumped back to where it was supposed to be. From then on it was seemingly correct.

I just checked my settings as well: ClimbPro and AutoClimb are turned off (I don't know the exact purpose of those, though). On the watch, the Altimeter is set to Auto Calibration = on, Sensor Mode = Auto.

I'm Using a Garmin Fenix 7 Solar.

Did anybody else experience a similar issue? I found the elevation correction feature on the garmin connect website. So I was able to fix it for the activity, but I'm still wondering what went wrong.

Thanks and cheers,

Stu