What are you doing wrong? Prb nothing, sounds like a nerve impingement 2 me. Translation -it’s just you. You can try physio or ART etc, but I personally would just avoid that movement from now on.
I’m the same (sorta) with squats, if I go beyong 90 degrees my hamstring tendon pops out and moves over my knee to a new position. Which isn’t too good under heavy load. My incredibly scientific solution? I don’t squat over 90 degrees. Yeah it sucks, but i think the tendon popping over load thing is gonna do damage if i keep on doing it eventually so I avoid it.
My point is, if it doesn’t feel good, don’t do it. If you really want to hit your brachialis, try reverse curls with a dumbell, or pronating hammer curls (pull up & rotate your arm so your palm is down, pinky out). Preacher curls have a reputation for causing elbow injuries, so it’s a good idea 2 stay away from them if they don’t feel good.