Garmin Express (7.8.0.0) stopped showing average pace from my Forerunner 935 (v21.00) in Activities

Do you mean in Garmin Connect?

Yes, Garmin Connect ( using Firefox 91.0.2 ) via Garmin Express on my iMac

In which page is average pace missing? Post web address or screenshot so we are looking at the same thing. 

Ok. In the dashboard. Does it work for other activity types?

Do you get pace in the activities list? https://connect.garmin.com/modern/activities?activityType=swimming&activitySubType=lap_swimming 
You might have to add the avg pace field to the page.

Do you get pace when you click the activity?

Swimming and 11.08 and zero pace - then there is already a thread about it: swim pace not shown in activity list in Garmin Connect Web.

As I wrote there, movingDuration got nullified in some JSON files, the same as maxTemperature earlier. Null is displayed as zero. On some pages it still works as it takes value from other link which still contains correct value.

No.