I’d just like to second what Charles is saying.
Dropbox sharing should work in Cubasis but does not. All other apps seem to integrate with no problems, so this IS an issue with Cubasis.
While the iTunes transfer workaround will work fine, it doesn’t solve the problem.
If Cubasis offers an option to share with Dropbox then it should work.if it doesn’t then it needs fixing.
Simply offering an alternative way of transferring the file is not a long term solution, and refusing to recognise the problem is counter-productive.