Looks like that mucky load of old bullets would cause a thick layer of scum to form and impurities would almost certainly be responsible for the cavities. On another subject, I´ve found a cheap, simple gadget to get the bullets out of the grease so that the grooves remain filled. It´s more or less just an old case with the primer hole drilled out and a spring loaded plunger to eject the bullet once it´s been stamped out of the layer of grease.