I have already completed the game, destroyed Radiance, so it's not like I could not get around that, but it still bugs me (no pun intended):

Using downward slash is kinda iffy.

That is to say - I have to press DOWN (and hold) some milliseconds before pressing the X key. The game does not seem to recognise simultaneous DOWN+X key press. And DOWN+C for downward dash.

Is it intentional design? Is it some artefact from controller scheme? (never played with a modern controller). Or a bug?

On the contrary, UP+X works fine simultaneously.

Inb4 I do have a gaming keyboard.

Does this issue happen with the same combination of keys on other programs?

(You could see if there's a good program for testing to see which keyboard inputs are recognized by the computer.)

Failing this, try either rebinding your keys or using a different keyboard (or using a controller).