Last week I did a 125km road ride with my 830 (recorded as road cycling) I just noticed that both in Garmin Connect and GCM the activity report shows some MTB related information !
This information did not report for the whole ride but started when I did a break after +-2hours in the ride, see the picture below.
Note that I do not have any MTB activity profile on my 830, I only have "ROAD" and "INDOOR".