I filed https://bugs.gentoo.org/show_bug.cgi?id=610388 with Gentoo. I since read in the errata for 5.9 (https://community.kde.org/Plasma/5.9_Errata#Wayland) that Wayland support for multi-screen is not yet finished. So this bug report is just FYI in case it will be useful to the developers still working on this support. Details can be found in the Gentoo bug referenced above.
*** This bug has been marked as a duplicate of bug 376377 ***