I have the same problem! My 2018 MBP can't seem to communicate with my Dell U3415W monitor reliably at all. Sometimes the monitor comes on when I touch the keyboard and everything is fine. Sometimes I have to "fake" a sleep cycle by shoving the mouse into the corner of the screen. And sometimes I have to go through a whole process of turning the monitor off/on, opening the laptop lid, disconnecting the Thunderbolt cable, and anything else I can think of. sigh it seems like this shouldn't be a problem in 2019.