maybe the internal screen is configured to be disabled?

I would like to support the oservatioss of the OP. I am running debian-testing (plasma-workspace 5.8.2) on a notebook (Lenovo T430P) with Intel skylake graphics HD 530. Behavior of plasmashell is say, erratic, if I connect an external monitor via Display Port (same resolution as notebook display, 2560 x 1440). Most of the time, I see the behavior described by the OP, independent on the notebook screen being enabled or not (I can still bring up a terminal on the external monitor and change display settings from there). In fact,disabling the notenook monitor keeps the last image on the screen.

Is this bug still happening on newer Plasma versions?

Well, I do not observe the exact behavior described in the original bug report, 
but there is certainly some unpredictable and sometimes truly bizarre behavior
still going on. I often see that the external display is activated, but then goes to sleep while 'taking' the controls (including the display settings menu) with it. If I am not fast enough to re-enable both displays, I need to re-attach or 
power-on power-off the external display to have a few seconds to see the settings menu in order to enable both displays. This mostly happens with displays which have already been attached. With 'new' displays, I tend to see the windows like selection menu which lets me choose cloned displays, external only and so on. 

Sometimes one of the displays still freezes when attached, but typically that situation can be remedied using CTL-ALT_F<less than 7> - CTL-ALT-F7 to kick the 
server/window manager/whatever needs a reminder a little bit in the b...

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New information was added with comment 4; changing status for inspection.

more details on one failure mode:
- Laptop T460p, Intel 530 graphics, discrete chip (NVIDIA) disabled, SW debian testing: plasma-desktop/..-data 4:5.14.5.1-4, libkf5plasma/plasmaframework 5.62.0-1, kwin-x11/common 4:5.14.5-1, linux-image 5.2.9-2 
- external monitor Dell U2713H attached via HDMI and docking station, configured
above the laptop display, resolution 2560x1440 native for both
- yesterday left this configuration after having disabled the external display, display switched off (soft switch, probably standby)
- this morning: re-attached laptop
- bring up systemsettings, the monitor shows up, power key shows standby (slow on and off light), activate the external monitor, apply -> all windows disappear, probably to the external monitor, monitor stays black.
- turn external monitor off and on again -> windows appear, but the background is black, no standard wallpaper, display remains static, but obviously input is processed (e.g. windows can be moved by mouse to the laptop screen, but the external monitor image is not updates)
- workaround: CTL-ALT-F<less than 7>, then CTL-ALT-F7 brings back the wallpaper and the monitor image is now updated properly

These are all different issues. :) It seems like the original bug is now fixed, so for anyone experiencing new or different issues, please file a new bug report. Thanks!