There are a couple of possibilities for why this is happening: (1) The game engine has some strange limit on how many and which keys are being held at the same time, rendering additional inputs as, well, not input. I imagine the result of this is not being able to shoot when aiming diagonally up-right. I am not well versed in why or when this happens, but it could be what's causing you problems. Alternatively, maybe it is (2) a hardware issue. If you are able to test your keyboard in another setting then maybe you'll find an answer. The newest version does not change how the diagonal shot works. Finally, I suppose there is (3) a small time window after pressing the shoot button, to aim, albeit it's very short. Optionally, you should be able to hold the direction in advance and then press shoot to shoot in said direction, although I doubt timing was the issue.
I did some quick testing, and I couldn't recreate the bug you experience, but I hope one of the points are of help.