FCK,
You're probably right, there is no telling what that primer body is made of. They may just be soft.
Was the primer flatten at all around the edges where it fits into the hole or was there still the original full radius from the face to the cylindrical portion? Also when you look at an unfired primer, is the anvil more or less the same size and shape as an American Primer, or is the space between the legs of the anvil more restricted than an equivalent American primer? If there is less of a path for the gas to flow it might be restricting it and it may be pushing the primer back more than on your other primers.
I know you have a lot of experience so bear with me here I'm just "brain storming" ideas. You've probably used the trick of opening up primer holes to allow you to shoot super wimpy loads, or even wax or other "bullets." If you restrict the primer flow without the corresponding set back of the case cause by recoil against the breech face the primer will push out. If the primer is self restricting it might have a similar effect.
Regards,
Mako