Problem with dual monitor config in Ubuntu 18.04

Your dock is probably not supported in linux unless its a true thunderbolt dock or has its own driver.

Please update your question with the full lspci output.

Thunderbolt 2 is the same physical connector as mini displayport , on a true thunderbolt dock the displayport is just another socket for the display card. I use an OWC thunderbolt dock this way , its almost invisible to the computer as its basically pci-e hotlpug.

If your dock is made by dell it almost certainly uses a displaylink chipset it will need a driver for it to work properly, you can get one here. It will be laggy and slow , these are barely usable in linux. http://www.displaylink.com/downloads/ubuntu

Unless you need the dock for the other ports i would just plug the monitor directly in to the thunderbolt socket, you might need an adapter.

This could be related to gdm3 using wayland. Please try disabling wayland for gdm3.

In order to make gdm3 use xorg instead of wayland, edit the file /etc/gdm3/custom.conf, find the line containing WaylandEnable=false and remove the comment sign (#). After you made the change, reboot and test if your problem still persists.