Long 106km MTB ride, training effect and zones way off

Hi,

I did an 11-hour MTB ride with an average heart of 140 and the training effect in connect (both web and mobile) is categorized as recovery.

Also, time in zones is way off:

Edge 830 rebooted itself 4 times during the ride (probably because of freezing temperature of -16°C). I did connect an external power pack to it, but it didn't seem to help that much. Edge is running the latest stable version 9.75.

Here's a link to the activity https://connect.garmin.com/modern/activity/13005117108

Anyone else experiencing similar issues?