Same issue here, New MS surface Pro 7, MS Pro Dock2, Dell U4919DW.
When I restart the computer I always need to reboot the display.
I always have to start computer then connect the Dock2 to get display to work.
Also sometimes screen gets blurry and need to reboot to solve.
Have lots off issues with our customer and dual displays not always starting on the dock.
I see lots of threads with people having problems but no real solution.
Our company has desided not to use Dock2 opting for Dell 6000 universal.
This is all hi-end stuff that needs to work.