I've been using my external display with the same cable since I got my 2018 MBP back 2 years ago. Today I put it asleep after closing out of Final Cut and came back to an issue where the video output seemed laggy. I rebooted the machine and reset the NVRAM/PRAM to no avail. It only happens when I have the computer in 'clamshell' or closed mode. If I leave the computer open while connected I can output 4K@60Hz but once I close it it switches to 30Hz. This is all according to my display itself.
2018 15" MBP, 560X GPU, 10.15.5 SU2, Dell P2715Q connected via a DisplayPort to USB-C cable.
2018 15" MBP, 560X GPU, 10.15.5 SU2, Dell P2715Q connected via a DisplayPort to USB-C cable.