As mentioned in #10, it seems that the issue is related to the dock.
When connecting a Dell P2417H monitor to the dock using the DisplayPort connection, I can replicate this issue. But if I connect the same monitor to the HDMI port on the P50 (instead of the dock), it's detected and I can use it as an external display (it always maximizes to the same monitor, but that's a different problem).
I don't have a Mini DisplayPort cable/adapter to test the monitor connected directly to the notebook's DisplayPort.
As mentioned in #10, it seems that the issue is related to the dock.
When connecting a Dell P2417H monitor to the dock using the DisplayPort connection, I can replicate this issue. But if I connect the same monitor to the HDMI port on the P50 (instead of the dock), it's detected and I can use it as an external display (it always maximizes to the same monitor, but that's a different problem).
I don't have a Mini DisplayPort cable/adapter to test the monitor connected directly to the notebook's DisplayPort.