Just to clarify: I've used control panel to attempt to set the default app based on extension (I can set Cubase 10, but it stays at Cubase 9.5). I've done Open With ... and set new default app. Nothing sticks. I'm on Windows 10. I've run Control panel as "pure" Admin, so it's either a Win10 issue only affecting Cubase 10, or Cubase 10 is somehow not correctly accepting the association. I'm so used to just double-clicking on the .cpr file that I'm going for Regedit next.Through out all the alphas and betas, making the C10 .cpr default instead of C9.5 was a problem for myself as well as others, but not everyone.
I'll assume you have done the normal "open with" Windows stuff with no luck?
This may or may not be a Steinberg issue. It could also be a Windows 7 problem. Have you googled the issue of wrong versions opening?