Just for my own notes. My office's Dell P2715Q display had the same problem below.
The problem has been solved, it was the cable that was the issue. Had a Clicktronic High Quality 4K DisplayPort cable but apparently that wasn't good enough, bought a Premium GOLD cable from Lindy which is DP1.2 certified and the issue no longer occurs.
The Gold cable only helped few month, then it started getting the same old problem....
Time to get a better 4K display. It's dirt cheap now ;)