Are you using the same device for all of your rides and the same “activity profile” (if it is a Garmin) for all of your rides?  Is it just some rides that are loading as MTB rides and others are loaded as “rides” randomly?  Is there anything you are doing differently between those rides such as selecting different bikes?  

Yes, a Garmin but I use 3 different activity profiles.  I got the impression that the allocation as mountainbike rides was random but I suppose it could apply to just one profile.  As I’ve corrected them in the past I can’t now go back and check which rides, but the most recent was my Race profile.  I am also selecting different bikes but that’s something that gets adjusted after the record is uploaded to Connect and I’m pretty sure that Strava gets the file as soon as Connect does so changing the bike doesn’t affect Strava.

Then it will be exactly the problem anchskier has suspected, one of your Garmin profiles has the “Ride Type” configured to “Mountain”. You can see in Garmin Connect in the list of activities too if mountain bike rides appear.

What gear you select in Garmin as default or after the ride doesn’t impact the ride type in any way, neither in Garmin nor in Strava.

Problem solved, thank you both.  Somehow my Race profile default ride type had been changed - I wasn’t even aware that there was a selectable default ride type!!