While I don’t have that particular issue, I have other issues related to external screens, with 10.14 and 2018 MBP: I usually run the laptop with closed lid and an external screen at work and at home; and use it in “laptop mode” during my commute. Now, sometimes (once a week?) when I plug it into the external screen, it just won’t wake up, unless I open the lid, and then it still won’t recognize the external screen. Have not yet found out a reliable way to resolve this, but I usually manage with turning the monitor off and on again and/or unplugging/replugging it, possibly a couple times each.

Much worse is that sometimes (once a month?) during this process macOS completely messed up all my open windows: they are all resized to a tiny size. And I have dozens or hundred of them open. This is horribly annoying, and didn’t happen to me before (in 10.11; I skipped 10.12 and 10.13, for reasons similar to the OP. Before that, I went from 10.8 to 10.11; before that, I never missed a release, often was running beta of the OS etc.; but starting with 10.9, too many issues made this undesirable).

I dread switching to 10.15; unfortunately, I will get a new MBP 16” from work, which normally I’d relish, except that there seems to be no way to use it with anything but 10.15. I’ll see if I can delay it until 10.16 is out and will pray that they started to prioritize bug fixes again, but I have little hope that’ll actually happen.

> I have other issues related to external screens, with 10.14 and 2018 MBP: I usually run the laptop with closed lid and an external screen at work and at home; and use it in “laptop mode” during my commute. Now, sometimes (once a week?) when I plug it into the external screen, it just won’t wake up, unless I open the lid, and then it still won’t recognize the external screen. Have not yet found out a reliable way to resolve this, but I usually manage with turning the monitor off and on again and/or unplugging/replugging it, possibly a couple times each.

I've previously had these problems, and they were made dramatically worse on Catalina. Each monitor has about 50% chance of correctly loading in, on what used to be 90%. My USB mouse (which is plugged in to the displayport monitor) also loads the incorrect drivers until I unplug and replug it.

Ah, I’ve never tried in clamshell mode - long harboured fear of the thermal envelope from the PowerBook days.