For me, if I go into the settings and reduce the resolution on my connected monitor, it usually works. But I also tend to switch to 1 output, displaying only on the DisplayPort through the dock, at which point the phone goes dark and the monitor which is in the bug state you described then lights up and works.
So, either my DisplayPort dock is better supported by the L5 or else more likely it has trouble with some DP configurations but not all.