I have this. The result of the shoulder capsule being smaller than most people’s. Have had problems with it since very young but didn’t know 'til more recently what was the cause. The pain gets exacerbated by lifting the saddle up on the horse, lifting it onto the upper rack in tack room, lifting if the shoulder is allowed to rotate inwards, much reaching up overhead with one arm to lift something off a hangar, etc. My mother had pain just below the shoulder on outer part of arm and was told it was bursitis (probably accurate).
A sports trainer told me the shoulder joint can be stretched out with exercises, but usually I have to be completely healed up to do such things as they’re easily aggravated.
It’s mentioned in Suzanne von Dietze’s book “Balance in Movement” as an example of how riders have different shoulder and arm conformation which affects the exact position of the elbows and hands. (i.e., rules are not hard and fast or precise for rider position).