I can see the thread is rather old, however maybe my answer helps somebody
I faced the same issue with KSP 1.12 from steam and OpenSUSE Tumbleweed 15.3/KDE and multi-layout US/RU.
Im my situation the issue was fixed with two steps:
1. Set keyboard hardware to what is actually used (I have 105 keys keyboard, was 101 in system settings)
2. Set default system layout to US, keeping US/RU variants with KDE switch. However, it may affect pure console sessions (frankly, i guess if somebody is using pure console with no X they hardly have steam and KSP installed...)
