Yes there's a (theoretical) loss of returns over the source of the clutter, because by definition the infill radar has to have a higher base of cover, so that it doesn't see the wind farm (or whatever's causing the clutter). But the loss is only a few hundred feet, and if that few hundred feet contains wind turbines several hundred feet high, there aren't many pilots who would want to fly in there anyway so in practice there's probably little or no loss of returns from aircraft that you'd want to see.
Don't forget it's been standard practice for yonks to simply blank out all cover over persistent areas of clutter e.g. road traffic, causing all genuine targets at all altitudes to disappear.
The difficulties with infill are less to do with lost returns than plots jumping as they cross the boundary of the patch, due to slant range errors between the two radars.
NS