Thanks for trying this - I'm a bit busy today, but that was going to be my next step; clean install of Linux in a VM.
I already did this; several times! It must be *some* setting or bad config file somewhere in my system, but it *only* affects KSP1.4.x. It's very very strange.
By default, KDE is setup to use a 101-key keyboard layout. My physical keyboard is a 105-key UK keyboard.
Changing the keyboard layout in KDE from 101 to 105 fixed it in KSP1.4.3.
Looks like Unity 2017 and/or KSP 1.4 changed the way how keys are processed considering earlier versions of KSP worked just fine.
Really Solved (?)
It wasn't the physical layout; it was the fact that I had multiple keyboard maps configured to let me switch between languages. Removing one makes KSP work. Unity bug or KSP bug?