I had the same initial problem with my M-5649-R1 302R LCA's. Took pictures of the issue and sent to Marty. He sent the new bushings overnight and that was that. I have not had an issue with the FRPP M-5649-R1 since.I ran those arms (M-5649-S) for about 6 months and experienced the exact same failure/poor design. There is too much clearance between the spherical bearing housing, and the bore on the end of the arm. There might be minimal play when first assembled, but as they get used the movement further exacerbates the problem. With that amount of play in a solid type control arm, the rattles resonates throughout the chassis and it really does sound like the rear end is falling out of the car. I was quite disappointed in them considering the source and cost, as mentioned above. Ford Racing feels I simply "out powered them" causing the extra clearance. I wholeheartedly disagree. I feel it's a design flaw, without question.
Having no desire to run an arm that makes more racket than my exhaust does, I decided to swap in FRPP's cheaper arms with the urethane bushings (M-5649-R1). Well, once again, FRPP failed here too. On every Mustang built from '05-'14, the gap in the lower control mounting point is different forward to rear, with the rear being wider. On the M-5649-R1, the steel sleeves are sized properly, but the arms and bushings are not. Since the bushings are a two-piece design, they simply spread apart after just a few miles leaving a large gap between them in the control arm bores which considerably lowers their effectiveness. I had to come up with a spacer to keep the bushings in place.
My disappointment in these arms is great. Not only do I sell Ford Racing parts, but I run them on my own car, and I push them hard. I promote them, I defend them, and except for these examples, I have faith in them. So I am very curious in what Ford Racing has to say in your case.