Is there an average pace when you view those same activities on the watch (Menu>History>Activities)? If the data is on the watch, then there's something odd going on with Garmin Connect rather than the watch. Either way, if it persists then raise a support ticket with Garmin so they can start investigating the issue.
This also happened to me yesterday, with Fenix 5, av pace still on the watch, but I can't see av pace on the app or web However I could see it yesterday..Just the day after it's gone...
I also found many such my runnings in GC. No paces there. And as i understood it looks like because our Russina time is without summer and winter time shifting. In GC all my workouts are 1 hour ahead of real time. And if i'm correcting time of workout manually its pace is disappearing. And it is strange, when i'm checking my daily heart rate chart, it is correct in according with time but all workouts of the day are 1 hour ahead of HR. PS On 5x the times of workouts are correct, so it is problem of GC