Also experienced this with various MBPs (both with dedicated GPU and integrated graphics) in clamshell mode. Disconnecting DP cable sometimes helps. But sometimes it requires opening the lid and using the internal keyboard to “wake” the internal display to from there resleep it and hope the next wake will wake the external display. And sometimes only patience (or remote restart) helps. It’s infuriating, especially considering it’s been a problem for so long and across so many macOS versions. It is indeed as if Apple simple didn’t care about fixing it. Perhaps external display = you’re viewing it wrong. 
That was the first thing I tried. It didn’t work - hence the need to reboot the computer.