Comment # 1 on bug 1226387 from Stefan Hundhammer

This appears to be a duplicate of bug #1225991. In that case, it was GNOME, but
it is entirely possible that the real problem lies deeper, so KDE Plasma might
also be affected.

Please read comment #11 .. comment #21 in that bug. Does that sound familiar?
If you experiment with different applications using different widget toolkits,
do they behave as different as in that other bug? What about xterm (ancient X11
Athena widgets), Firefox (Gtk), the YaST Qt control center (pure Qt)?

Does using the KDE Plasma keyboard switcher applet change the behavior?