In xwayland, for example, running plasma-desktop the plasma-desktop panel still appears with a full titlebar, and is not positioned on the screen where it should be.
Right clicking on it, the submenus are also placed wrong (unless you click on the VERY bottom of the panel) but less random. The child submenus of the panel menu however are placed in the correct area
xwayland is supposed to allow regular X clients to run under wayland, but it's not intended for desktop chrome style clients (panels, pagers, etc). Those applications are not expected to work correctly.
Either port the panel to be a wayland client and use the weston desktop-shell interface, or just use the weston panel.