My guess is that over time the grip screw had been over tightened to the point that the plastic just "let go."
CaryC (Cary Chapman Custom Grips) provides a spacer with his grips to avoid this. As to replacement, I'd contact Cary. He offers about the most beautiful grips anywhere. He offers stabilized wood and oil-finished woods.
I've had a number of his on my guns, and have high regard for them.
Here, one of my .45s with Cary's walnut grips:
If you want the ivory look, Cary offers ivory Micarta. Ajax and Eagle grips also offer genuine ivory, fake ivory, and stag.