Activity paused without intervention and suddenly

Have you checked Autopause and related settings?

Yes, I don't use it, I have it disabled

I use my 6S for 15+ activities per week and haven't had this issue.  Have you tried powering the watch off and back on?  Maybe also try turning autopause on and then back off.  I also suggest diligently locking your keys/buttons or using autolock to do the same.  This will eliminate any chance that somehow the stop/start button is being pressed during your activity.

I have had my Enduro for two weeks and every single time my activity gets like totally called off itself, when I realize I am in the middle of the track and the watch is frozen or in the regular time screen without even running the mtb functions, this is frustrating, some other times the watch screen freezes like after half a kilometer after starting and of course I cannot be paying attention to the watch all the time, unfortunately I have to use my phone again to follow trails because I just cannot trust my Enduro... I am sending it back also due to a green stain in the back of the screen, I hope the replacement is free of these issues, because I like my watch... :(

Have you considered you may have press the start/stop button (upper right) by accident ? I happened more than once to me, when  moving, to have it pressed unintentionally. Now there is a button lock function, I have it programmed to be a long press of the start/stop, I don't rementer if it is the default or I changed it. When leaving on an activity I most of the time lock the watch so unintentional button presses are not registered. 

I find that difficult since it requires a combination of undesired button presses to go to the screen and totally show as no activity not even a section, there wa one time when it showed 0.62km traveled distance when I did more than 20km, the elevation recording was ok… I have had very good response from Garmin so far and I am sending it back…. I hope it comes home ok

I have the same issue twice in the last 2 days (biking). Screen shows à dented wheel 

I have had the same issue twice in the last 2 days (biking). I think that the screen showed a dented wheel at some point. Didn’t restart until I restarted it  

1) can be auto-pause if you have that enabled. If you don; t want it turn it off.

2) It can be that you press the button by accident. When in activity I lock the device so this cannot happen. There is a lock option in the controls, and on my watch I have it assigned to "long press start button", I use it all the time