Aimpoint's ACRO leaves a small gap between the sight and an in spec ACRO footprint or plate. The void is the result of the locking bar slot on the plate being deeper than the locking bar below the sight. The gap is normal, and can be found below an ACRO P1 or P2 and an in spec ACRO plate, milled slide or mount.
An ACRO P2 mounted on a B&T Picatinney adapter:
Below is an ACRO P2 mounted on an OPF-G, ACRO plate: