Currently plasma-desktop installs plasma-workspace which includes wayland support (/usr/bin/startplasma-wayland) but doesn’t include kwayland-integration as a dependency which causes all sorts of weird behaviour with plasma widgets. So kwayland-integration should be included in plasma-workspace dependencies.
kwayland-integration is a dependency of plasma-wayland-session, which you should have, if you log into a Plasma Wayland session anyway.
I didn’t install plasma-wayland-session because I use greetd as session manager and use it to launch /usr/bin/startplasma-wayland (provided by plasma-workspace). If plasma-workspace provides a plasma wayland launcher it should install complete support.
I think the logic is (from Plasma devs) that you should use the provided plasma session package, if you want to have a good experience when running a Plasma wayland session.
But you are trying to do it your own way and you found out you where missing a dependency for “full” functionality. So install that and be done with it.
Weston provides a wayland session, but that is no where near complete support. 
If that is the logic then plasma-desktop should include plasma-wayland-session as dependency (given that plasma-workspace includes startplasma-wayland). Is not just “full” functionality, missing kwayland-integration make it looks like plasma bugs (volume popup not centered, broken widget moving in panel, etc.)
In any case, you would have to make your request to the Arch Linux package maintainer, because that’s where we get the packages from. And he is probably following the dependency tree laid out by KDE devs.
Well it is Plasma wayland bugs. Not Plasma bugs, since the bugs won’t happen on X11 session.