GPS problem on fenix 6x pro solar

How much higher was the speed? Are you doing the same type of activities and in about the same location and how often since 20-10 did you see inaccurate tracks? Has it been raining during your activities? 

I was running at a Z1 pace, but the watch showed a Z5 pace. Yes, I was running on the same track as always, with an open area and clean weather. I'd never had any problems with the GPs until then. 
I already did a soft reset during the workout, but the problem persisted. 
Days after I did a hard reset and to my surprise the problem persisted in the following workout.

Unless you dropped the watch or water got in it and something inside is damaged, there is no obvious cause as far as I can guess based on this. Sometimes atmospheric conditions at altitude can be a factor, you won't necessarily notice at ground level. You could check here: https://gpsjam.org or here: https://gpsoutage.com/gpsaccuracy.
A full reset and syncing with Express might help.