I am having a similar issue and figured I would share the details here as well to see if it can benefit.
I am running a 13" MacBook Pro w/ Retina display (Early 2015) using a Targus DOCK160USZ hooked up to an LG UB3467-B monitor. The setup itself I know is working because I have tested the same configuration with windows and I have no issues whatsoever. in addition, when I plug directly into the Mac I am able to attain the max resolution of 2560 x 1080 with no problems whatsoever.
In the interim, I have had to take a couple of steps to help "mitigate" my issues.
First, I went to my local Best Buy and bought a brand new 4KUHD HDMI cable and replaced the HDMI cable I had in place. Second, I lowered the display resolution of my monitor to 1080p.
While this is helping for the moment, I am constantly reminded of the issue by having the vertical black bars on either side of my desktop.
You guys are doing fantastic work and I know you will have the bugs resolved soon, but in the interim, the frustration is certainly there
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