I'm not an expert but I would assume that some grip frames and foregrips just don't match up the same, tolerance wise. I think I have seen pictures of frames that you couldn't even fit a piece of paper, and then frames with gaps like mine. I honestly think the dovetail that the foregrip slides in to is different on each individual frame.
I took the foregrip off the part swapped gun it's on and put it back on the olive frame it came with, and tightened the screws like I normally would. Look at the difference in gaps. Almost no light coming through