I have difficulty answering the question, as my diving is so varied. While I teach in challenging visibility conditions, I use a hand mirror and lights on my students (dive and tank - so I can keep track of them out of the corner of my eye) to help me keep track of students (I prefer to keep rations of 3:1 or even less. 1:1 in some cases). When I dive tec, I also never lose my buddy. When I'm on a challenging drift dive, again never (which can be hard when going through the Tacoma Narrows).
But when I have a camera, and the conditions are calm, it sometimes happens. I was diving with a group, and when I started recording two octopuses fighting, everyone else swam off. My ex-girlfriend who got me into diving would completely ignore everyone when she had a camera in her hand. She never looked up. I had to stay close, but I still feel that falls under the case of buddy separation. When I have a camera in my hand, I feel that I'm solo-diving with a buddy. While I maintain eye contact (I pause to check up on my buddy in between shots), I can still lose him/her. If I buddy up with a beginner, then I'll just take my GoPro and leave the FF DSLR in housing (the "beast") at home.
So it really depends. As others have said, communication is key (before you get to the dive site).