If you see a slight gap between the RMR and our RMR plate's rear fence (many of them have one, some don't), the small gap is normal.
When we developed the plate in 2019, we noticed the gap between an RMR and OPF-G, RMR's rear fence, we we wanted it eliminated. In talking to the shop (and several times after the launch of the Glock MOS RMR plate in March of 2020), I'm glad they talked us out of it.
RMRs have a length tolerance of +/- 0.002. The distance between the center of the lug holes to the rear has a tolerance of +/- 0.008. Had we made the plate in a way that eliminated the rear gap, the plate would fit some RMRs, it would be too tight for other RMRs.
Our RMR plate design uses the lugs and front fence to pinch the sight in place and hold it immobile in recoil, the rear fence is only added to fill the gap between the sight and the slide, and is otherwise not functional.