XPS 9560 + WD19TB + Multiple Monitors

@WesC88  The XPS 15 9560 is a Thunderbolt system that supports DisplayPort HBR2, not HBR3.  When a 4K 60 Hz display is in play, the WD19TB has some cabling restrictions on how displays can be connected if you want all of them to operate at their native resolution and refresh rate.  These restrictions are related to how the dock allocates display bandwidth coming in from the system, and the restrictions are different based on whether you're using an HBR2 or HB3 system.  This is spelled out in the WD19TB User Guide on support.dell.com, specifically in the Display Resolution Table.  The short version is that with an HBR2 system, if you have a 4K 60 Hz display and anything else, then one of the displays has to be connected to the "downstream" TB3 port at the edge of the dock, using something like a USB-C to DisplayPort adapter/cable.  The other can be connected to any other output.  You currently have both displays connected to the "core" display outputs, and with an HBR2 system, half of the available display bandwidth is allocated to that downstream TB3 port, with the "core" outputs sharing the remaining half.  And that half isn't enough for 4K 60 Hz plus what I assume is actually QHD/1440p.  (On that note, "2K" is a dangerous term to use because so many people use it incorrectly.  It's actually a film resolution of 2048x1080, so in the consumer space it actually refers to the nearest and slightly LOWER equivalent, which is 1920x1080.  This is just like 4K, which is a film resolution of 4096x2160, but in the consumer space it refers to 3840x2160.  If the resolution you're calling 2K is actually 2560x1440, that is rightfully called QHD/1440p.  The former is only one extra character to type over 2K, but it adds a lot of disambiguation value, and disambiguation can be important when dealing with technical issues. )