SUMMARY
Plasma/Wayland doesn't recognize external display on reconnected USB-C dock.

STEPS TO REPRODUCE
1. Use a type-c USB w/ alt mode laptop and a dock (like Dell WD15)
2. Log in to Plasma Wayland from SDDM with the dock connected, see external screen work right
3. Disconnect the dock, reconnect some time later

OBSERVED RESULT
External screen doesn't activate (but everything else does, e.g. sound, USB devices, ethernet work well on reconnect)

EXPECTED RESULT
Screen should be identified and turned on

SOFTWARE/OS VERSIONS
Operating System: KDE neon Unstable Edition
KDE Plasma Version: 5.16.80
KDE Frameworks Version: 5.62.0
Qt Version: 5.12.3
Kernel Version: 5.2.0-050200rc7-generic
OS Type: 64-bit
Processors: 12 × Intel® Core™ i7-8750H CPU @ 2.20GHz
Memory: 15.4 GiB of RAM

ADDITIONAL INFORMATION
- This works on both Plasma/X11 and Gnome/Wayland.
- As far as I can tell, docks generally use DP MST to connect multiple displays, even HDMI connection is served via DP MST. So this might be an MST-specific issue (reconnecting works with direct HDMI on the laptop)
- Tested with Intel gfx
- I see the port appear on /sys/class/drm/card0-DP-6
$ cat /sys/class/drm/card0-DP-2/status
connected
$ cat /sys/class/drm/card0-DP-2/enabled
disabled
It seems that the display never gets enabled, even though it's found.

While searching for a bug report on this, I only found a forum entry - same story, but with HP laptop + dock: https://forum.kde.org/viewtopic.php?f=111&t=156876