I have this exact same problem - 2018 MBP, Dell U3415W. Sometimes it works, sometimes I can move the mouse enough to get it to work, sometimes the monitor won'

I have this exact same problem - 2018 MBP, Dell U3415W. Sometimes it works, sometimes I can move the mouse enough to get it to work, sometimes the monitor won't wake up at all, sometimes the monitor wakes up and has a VGA screen resolution. I thought I had "fixed" it by turning off sleep mode in the MBP, but after a couple of days of working properly it started failing to wake up again. I know its a problem with the macOS somewhere but Apple just doesn't seem to be focused on fixing these types of things. Even if you file a bug report it disappears into the ether and is never heard from again.