I play on controller and have been experiencing the same bug as well. I tested this in the Simulacrum and had the same results as you posted. Melee(B), hold LT, continue combo, let go of LT, continue combo pressing B, combo ends, let go of B, Frame goes ADS (Until giving an action like shooting, jumping, tapping LT, etc, to stop the ADS). Interestingly, if you continue with a neutral combo (after your Hold LT combo) for example, it can also cause the same bug, you will ADS at the end of the combo as well.
Just some extra info, but I tried it with "Hold Aim" option toggled on and off. Both caused the bug to happen. Hope this helps.
