Flexing ring finger cause bicep to cave inwards

By flexing just my left ring finger (no effect from other digits) my bicep will sink down towards my triceps, an when Done with my arm relaxed by my side it will move my forearm forwards slightly.

Doesn’t occur with index, middle or pinky fingers, or if I use all my fingers at once. I went to the doctor an they said it was probably a muscle cramp, it’s been several months an it still hasn’t changed.

I have no pain an no recollection of injuring my arm, I’ve tried ulnar nerve flossing since it cause my arm to move forward but hasn’t changed the symptoms. Thank you for your time hopefully it makes sense, not sure how to post videos but that’d be the easiest way to understand what’s going on.
James