Posting here as well as Fenix 5 plus forum as I believe the issue lies with GC, not the watch.
The problem I have found is that GC has interpreted swim drill interval data and included average speed and it never does this. Drill sets are always given a 0 or blank (-:--) for average, so that it doesn't affect overall swim average. This is also passed onto to 3rd party apps such as Strava which uses the drill pace.
Just checked the intervals on my watch and it actually shows the drills correctly with a blank pace, so it appears to be a fail with GC.
I am running Android 9 and GC version 4.22.1 - however I am unable to find a change log for this release to see what might affect it. Most recent swim I have that the data looks good on was 28th Aug (which I think is a day before 4.22.1 was released).
Can anyone else confirm this has happened to them also?
Thanks!