Fenix 5 PLUS drops out of the activity after 9.00 firmware update

Same here.. 

I downgraded to 8.40 (it also resets all settings on a watch) and looks like it has fixed the issue. Will go for a run in few hours and report back

So went for a run and when starting the activity, discovered, that watch had updated to 9.00 again. Apparently forgot to disable auto-updates after downgrading to 8.40. Anyway, the run went well, no skips to watch face this time. It might be that resetting the watch could help in this situation. 

Haven't seen this myself; done three runs, two shorties and one 25k on 9.0 and haven't had any issues.  Hope the reset was all it needed!

Have done 2 45 minute walks since update and all working as expected for me with no dropouts so far

I concur, 1 x ourdoor run, 2 x indoor workouts since 9.00 update, no issues whatsoever.

I had this issue with 8.40. I speculated that it had to do with my custom watchface. Strangely, the watchface shown during the workout was a built-in one... It also started with having activated auto-update for the ConnectIQ apps etc. in Garmin Connect Mobile (Device > General > Auto Update Apps). I deactivated that setting in GCM and ever since I haven't seen the issue come back. I think I fixed it this way.

I had this issue with 8.40 as well. I thought it was related to a custom ConnectIQ watchface or a widget autoupdating, so I turned off autoupdate.

Never had it since. Now I update the ConnectIQ stuff when I synchronize.

I had this issue this morning and I was really annoyed. 
for the folks who solved it: its enough to disable the auto update feature for the watch faces and the widgets in the iq connect app?

thanks.