When opening split view mode (F3) then closing it and reverting back to single view, it's often impossible to determine which of the two views will be retained. In the past, the view that was selected used to be closed... since an update a while ago however, this isn't always the case and which view remains open seems to be subject to random chance.
Example: Let's say that in the left view, you are browsing the directory "foo/bar/a" while in the right view you have "foo/bar/b". You press F3 to close the split view. Will Dolphin stay at "foo/bar/a" or "foo/bar/b"?
My personal suggestion would be the opposite of what seems to have worked up until recently, as that feels the most intuitive; Keep the view that is selected open, and close the one that is not selected (is gray). The user is drawn to expecting that the view they're currently browsing will persist, so that's likely the best choice for Dolphin to take.
*** This bug has been marked as a duplicate of bug 312834 ***