[NAME]  The XPS 15 7590 is a DisplayPort HBR2 system, not an HBR3 system.  And if you open the WD19 User Guide on the Dell Support site, you will find that with an HBR2 system, your dual display max is 1920x1080 (technically 1920x1200 works), and you can only run 2560x1440 in a single display setup.  This is due to bandwidth limitations of HBR2 and regular USB-C.  You'd need an HBR3 system to run dual QHD 60 Hz through that dock.Moving one display to the built-in HDMI output on the XPS works because in that case, that display's bandwidth requirements don't have to be met over the USB-C link.  But if you want both displays connected through a dock with your system, you'll need the WD19TBS, which uses Thunderbolt 3 rather than USB-C.  When paired with your system, it will be able to tap into 4x as much bandwidth as the regular WD19, allowing it to run dual displays up to 4K 60 Hz or even triple QHD (if you disable the built-in display, since you can only run three total displays from that system.)
Hope someone can help. I just got an XPS 7590 with a WD19 USB-C Dock. The 180W model. I have 2 x LG 32GN600 monitors. Can support 2560 x 1440 resolution. I have Windows 10 with all the latest drivers and firmware installed. When I have one monitor plugged into the dock I can run that internal monitor at 3840 x 2160 and the external at 2560 x 1440. But when I connect the second monitor I cannot get them to go higher than 1920 x 1080. This all is at 60Hz. I am able to get both monitors to run at 2560 x 1440 with a 2019 macbook plugged into the same WD19 dock. What more can I check or what setting might be stopping me from running the 2 monitors at 2560 x 1440 like the macbook. Thanks Rudolph      
XPS 7590 / WD19 cannot get external monitors to run at QHD