When casting Abathur’s Stab by holding down the Q button, subsequent stabs will fire at the original location. Every time Stab is used, it should fire towards the direction of the mouse, regardless of if Q is being held down constantly, or tapped repeatedly.
Reproduction steps: Abathur Stab Doesn't Follow Mouse - YouTube
Note: This hasn’t always been the case, and feels like it was introduced recently (past few months).