I'm glad you liked it, and congrats on beating all of the bosses!!
As for the issue with the forward attacks, its the first time I've heard of that issue. the only thing I can think of is that, when you would try to attack forward, you would drag a little bit above or under, which the code recognised as an "up" or "down" attack.
The game detects the angle of your mouse drag and uses that info to detect the attack type, so that might be whats causing it.
So it might just be a little quirk of the game, lol.