Hi I had the same symptoms and found a cause that I want to share with you.
For me, the cause was keyboard layouts. When I had German T3 or German E1 enabled or even only in the list of active layouts in Plasma, I observed the described behaviour.
I had this configured, because it is given as a workaround for neoqwertz layout on https://www.neo-layout.org/Probleme/FAQ/#linux-unix-bsd
The third workaround option (setting modifier_mapping) had the same effect. Cure: have only one keyboard layout "German" configured in KDE Plasma. Regards Philipp Schwarz
