From my testing, the auto-attack that breaks the stealth from Hidden Thief always occurred when I was wielding a ranged weapon (pistol or bow) in my main hand, and never occurred otherwise (sword or dagger in main hand.) It also only occured when my weapons were already drawn. (drawing weapons to start combat negates the errant auto-attack.) I can’t for the life of me think of a reason why this would occur, or why you would want it to. I just hope they remove this functionality soon.