Hello, again~ I've investigated this some more, and it seems that it's a bug with the underlying library that MasterPlan uses indirectly for windowing and input, GLFW. It seems that the function keys / non-printable keys (Control, Escape, Space, etc) don't work properly if you switch to a secondary keyboard layout. It doesn't seem like it happens if you're on the primary layout, regardless of what layout it might be?
I'm somewhat perplexed as to how / why this is the case, but I am able to reproduce it on my side if I start MasterPlan while using a secondary layout, so at least we know why it's happening.