Thank you for getting back to us with the additional information!
My original theory was that you had a 13-inch model laptop with an Intel graphics adapter only,
Intel graphics adapters in the MacBook Pro can only support one display at 5K resolution, so the timing of the connections of the displays could have been influencing the behavior.
To put that another way, if the 5K display was connected to the system first, then system would not have the capability to drive a second Samsung display. If the Samsung display with 1920x1080 resolution was connected first, when the LG 5K display was connected it may work but at a lower than maximum resolution.
Of course, since you mention your MacBook Pro is the 16-inch model that has both an Intel graphics adapter and an AMD graphics adapter (https://support.apple.com/kb/SP809?locale=en_US) that limitation does not apply here.
By any chance are you connecting the Plugable dock and the LG display to Thunderbolt 3 ports on the same side of your laptop? If that is in fact the case, if you connect the LG display to one side of the laptop and the Plugable dock to the opposite side of the laptop does that change the behavior?
I ask because there are two separate Thunderbolt controllers within your laptop. One controller is connected to the two Thunderbolt 3 ports on one side, and the other controller is connected to the two Thunderbolt 3 ports on the opposite side.
As a result, I am curious if ‘separating’ the devices across the two Thunderbolt controllers changes the behavior in any way.