I have an android. Today I did 3 activities and none of the milage worked on the watch.
I was on the treadmill and selected treadmill activity and it didn't sync. I then used the run activity and it didn't track either but maybe that is because the run activity is solely for outdoor
I then did a run for Garmin coach and the mileage didn't track on watch.
The intensity and steps counted though.
I have ran on a treadmill this weekend twice and the mileage worked on watch. Not sure what happened today.