I got the different vintage 15 bobbins, and they work! Meaning they don't disengage when winding. I am so excited! But now I found they work, I want to help other fix the same problem.
The bobbin in the picture with 1 circular hole is the kind that works.
The bobbin with multiple holes disengages doesn't work.
Note:
My machine is a 1947 15-91 Singer made in the U.S.A. My conclusion is there are different class 15 bobbins, some fitting in some machines, some in others. All I know is the bobbin with 1 circular hole fits my machine, and the class 15 one I have with multiple holes doesn't.