Since 2 days ago kde apps (dolpphin, gwenview and konsole, for example) run under xWayland instead natively under Wayland. "libraries" tab in "about" dialog says that the window system is xcb.
This is intentional, see https://blog.martin-graesslin.com/blog/2018/03/unsetting-qt_qpa_platform-environment-variable-by-default/
Thank you for the info Antonio.