I have had the same identical problem!
Garmin Fenix 5x plus, running initially 7.10 firmware now I’m on 7.52 beta! Issue was present with both firmwares!
Iphone X running 12.3.1
Not only do the widgets rearrange to the default order but the ones I uninstalled appear right back and the ones I installed (such as notifications) get uninstalled!
At this time I use NO third party widgets!
I spoke to Garmin and they are looking into the problem!
I was even able to catch the approximate 2 hour timeframe during which this happened and send the recent app logs and sync logs to them!
Quick question since our hardware setups seem similar:
Do you ever experience moments in which smart notifications do not get to the watch even if they do get to the phone?
Do not disturb is OFF on the watch! That is both global DND as well as DND while sleeping...both OFF!
The solution to your problem is to not make adjustments to your Activities (adding). I realized that once I left my Applications alone and adjusted my Widgets, then all was fine. The problem is, that you can’t really try out new Applications without it affecting your Widget loop and causing it to default. So, set your Applications as you wish, THEN set your Widgets.
Not the proper answer that you want to hear but a solution nonetheless.