Wheel Cylinder Issues!

1982 Chevette Diesel - just replaced the rear brakes and found that the wheel cylinder pistons are not centering on the brake shoes. They are off center enough that if the pads aren't adjusted snug against the drum, a hard stomp on the brakes will cause the piston to slip off the shoes and out of the cylinder.

Is this common? Was there a mid-year switch on parts for this model maybe, and I'm not getting the correct parts?

Any insight to the issue or advise is much appreciated!