I know this is an old thread, but I’m wondering if the original poster ever found a solution. The reason I’m asking is that I am now having the same issue after having updated from Cubase Pro 11.0 to Cubase Pro 11.0.10.
Just to clarify a little bit: If I change the association to Cubase 10 in “Choose default apps by file type”, it does open Cubase 10. But, if I change it back to Cubase 11, it opens Cubase 9.5.
Rick