Alienware 15 R2 - Discrete graphics not working over DisplayPort USB-C connection.

My 15 R2 has an AMD R9 395X discrete GPU.  When using just the laptop itself, I don't have issues with the switchable graphics, the Intel 530 works when it should and switches to the R9 when gaming.  When I connect an external monitor (Dell U2415 Ultrasharp) via a USB-C to DisplayPort (1.2) Cable to the monitor, it only uses the 530 graphics - even for games in High Performance mode.  In games where I can select the GPU I want to use - only the 530 shows up, and the AMD Settings states that there aren't any displays connected to it.  Both drivers for the respective GPUs are the current versions as posted by Dell.  

Is this a known issue that is being worked on, or should I be opening a support ticket with both Dell and AMD?