Dell Dock WD19 or WD19S users may find that the power button LED turns off and stops functioning when they attempt to undock and immediately re-dock to XPS 13 9300. However, it does not impact the USB2, USB3, DisplayPort, HDMI, LAN, or Audio functionality.
This is working as designed. Dell Docking Electronic and Communication (EC) delays for a period of time for USB Hub to begin initialization when you attach or detach the dock to the computer. If you detach the Docking Station from the computer and connect immediately, the docking station fails to initialize and does not function properly. This applies to all Dell Docking stations.
Remove the dock and wait for a few seconds before reinserting it into the computer.