I find it hard to parse the change logs for versions in upsource as the versioning number is changed and there's no place with a concise text list from version to version so I'm not sure if my problem has been fixed or not.
When I select a branch in upsource 3.5, it only tracks up to the last merge. When I select a commit it only creates a review for that commit.
There are various potential work arounds such as allowing review from compare branch root to branch leaf (although you get merge data and lose branch tracking), or to specify a commit further back (manually select branch root, a bit better but you still get merge data).
the thing is what most people are really going to want is to compare against the branch they expect theirs to be merged with, where you would typically merge that branch into yours first so that the merge into the target branch will be seamless. You can also select this with compare but again can't create a review.
There are logical ways to fix this as well but they're pretty complex. The easiest thing would be to just focus on what the user wants to do, which is present their branch as being worthy to be merged with trunk or master for example.