Hi Michael,
This is a little puzzling,
I am not a fan of the longitudinal split diagnosis for tendons in the upper limb and would need some convincing. In the lower limb weight bearing tendons (tib post and peroneus brevis as great examples) I think longitudinal splits are common, but in the upper limb I am not so sure.
I agree there is a little fluid around the tendon indicating a small joint effusion although it is really at the border for being physiological for me.
The long head of biceps does have some anatomical variations possible and what you are seeing might represent an anatomical variation. Interestingly when I see anatomical variations they are often bilateral for some reason so look at the other side.
Anatomically the collagen you are seeing just anterior / medial to the main tendon is in the area where I expect to see the coracohumeral ligament and perhaps thi is a little thickened. This can be associated with cuff injury or adhesive cap.
I would follow that other collagen area and try and determine if indeed it is the coracohumeral ligament.
another possibility is thickening of the sheath itself.
A few things to consider!
Steve