This is normal with people who deadlift using a mixed grip. When guys go learn posing to get on stage they have to consciously learn how to adjust their bodies to mask this. Just make it a very conscious habit to force yourself to raise one arm a bit higher when you flex, and in daily life you can consciously remind yourself to relax the trap on the side that sits higher until it becomes an unconscious postural thing. Takes a few weeks but it does work. You can try stretching the trap on the raised side (just look up stretches for it) and have someone use one of those massage guns on it or try and position yourself against a wall so you can roll it on a tennis ball or golf ball, this is all pretty annoying to do though and the traps are incredibly difficult to actually lengthen and release with any of these techniques.