I'm noticing a bug (I have a Fenix 5 on 12.51, iPhone 7 Plus on 12.1.3)
Go to Applications and add/delete one - The Widget list then gets reset and synced back to the watch and next sync
I have the same problem. Vivoactive 3 Music v 4.0.0.0, iPhone 8 iOS v 12.3.1 but previous versions of both devices had the very same problem. Garmin restes widget list on every application change, like adding/deleting app, updating app, configuring app from Gasrmin Connect. I found no workaround at the moment.