The last couple of weeks my Fenix 5 have been way of when showing pace on the watch.
If i run at 4:30 min/km it might say i'm going at up to 6:30. Sometimes it does show a accurate pace, but often it does not.
The pace it gives me for the lap in total seems more reasonable and it is also more accurate when uploading to Garmin Connect.
I don't know if it is due to an update, but it has only been like this for about a month.
Resetting the watch, setting GPS to 1 second and using both GPS+Glonass/Galileo does not solve the problem and Garmin have no solution.