Happens constantly, especially in Abyssal Dungeons. Attacks always stop on the first or second attack at random or the movement completely halts when holding the button down to move. This needs to be fixed before the even harder stuff comes out because it 100% can get you wiped in those raids.
Tested this with a Zowie EC2-B, a Razer Deathadder, and some budget mice lying around. Happens with all of them. This issue isn’t in the KR or RU version of the game.