It is possible your OAL is to long, allowing the bullets to hit the rifling, driving the bullet back. Check the nose on a few this has happened to to see if there are any rifling marks.
I assume you are using a taper crimp, as a roll crimp is not suitable for use in a semi-auto cartridge that headspaces on the case mouth. It is possible that your resizing die or crimp die is a little on the loose side, allowing the bullet to be pushed back into the case - something you might not experience with the heavier lead bullet do to increased surface area and possibly a .001 larger diameter with the cast slug. Try applying a firm taper crimp, then push the tip of a loaded round against a hard object. If it resists going deeper into the case with hand pressure, then the crimp / bullet pull should be OK.
Larry