SUMMARY
KMail interacts very strangely with other applications under Wayland, and seems to "fight" with others about drawing on the visible screen.

STEPS TO REPRODUCE
1. Install KDE neon git-unstable, start a Wayland session.
2. Start KMail. Have a KMail main window on the screen.
3. Start Spectacle.

OBSERVED RESULT
When mousing over the spectacle window, sometimes the KMail window is repainted over the spectacle window. Then when mousing over input elements in Spectacle (e.g. dropdowns) those are re-painted on-top of the bits of KMail.

Same happens with Quassel client, but also KMail message composer windows which will be obscured by the painting of the main window (behind).

EXPECTED RESULT
That KMail on Wayland behaves like on X11: the main window isn't painted overtop of other, active, windows.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: KDE neon git unstable
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 5.54
Qt Version: 

ADDITIONAL INFORMATION
Screenshots are kind of hard to make of this; I suppose I could do a movie with an external camera. I've only tested this on a Slimbook, i.e. Intel graphics.