Fibers of lower traps run sort of across and in opposite direction to the fibers of pectoralis minor. Pectoralis minor pulls on the scapulae and rotates it inward, internal rotation, and downwardly. This puts the glenoid fossa, where the head of humerus rests more forward. This forward position, if kept in bottom of the bench press, causes the humerus to travel back a bit extra, if we want to reach the bottom of bench press. That extra humeral motion puts strain on the anterior shoulder, leading to instability, strain and eventually pain. To combat it, either improve how much you can shorten the lower traps, as you might be able to to shorten it a bit, and even be strong at the longer length, but pretty weak at shorter range. So if you can easily do the YWTL trap exercises, but you struggle with wall angels, then your LT is strong at mid to long range, but not so in shortened. Improving that might be the solution to the anterior shoulder pain. Additionally, especially initially, restrict how much you lower in bench, find the spot where it starts feeling uncomfortable, and stop just before reaching that spot. When your LT improves, you might be able to go lower, if you want to.