FYI: My investigation turned out that it was indeed an upstream problem of libinput. https://bugs.freedesktop.org/show_bug.cgi?id=105830 So this report against xinput can be closed as invalid/upstream.
- Bug#894325: xinput fails to set-button-map, no matter whe... Stefan Blachmann
- Bug#894325: Bug is deeper, not in xinput, but in lib... Stefan Blachmann
- Bug#894325: Bug resolved - upstream issue. Stefan Blachmann
