You can see one of the tabs remains...and actually, it looks like it was positiioned OK...unless it rotated? Could be a simple case of "got a bad one"? (C clip I mean ) Or it might have been damaged during installation?

This reminds me of an incident I had with one of my Webra .50's..
I bought a couple of new wrist pins, and one of them was a bit on the long side...about .020" and that prevented the retainer clip from getting down fully in the groove in the piston. (I didn't use that wrist pin...sent it back )