I know because the demo bike from 'ONYERBIKE' had the same problem but with the right mirror. I think someone had tried to adjust it by moving the housing rather than just adjusting the glass. I moved the collar down to see if it was a screw that needed tightening but since it was a rivet left it alone.
I'm glad you are enjoying your Strada, and the Cat and Fiddle is the right sort of road for it. I have an ST4S which has been modded by John Hackett and is a missile, but I am seriously thinking of changing to a Multistrada-it is just so much FUN to ride. Trouble is by the time I add the panniers and top box, hugger and front guard extender, hand guards and termi's I reckon it's going to cost me some serious money. And that's before you start to mess about looking for more power( I know, I know, I shouldn't but I always do!)
John