the rf patch brightness and the frameline brightness are controlled by different things, the frameline masks are lit by the frosted white window so it's possible that something is blocking the window (or it's dirty). the dim rf patch could have been caused by poor repair work. damaging the small mirror that reflects the image into the beamsplitter for example will cause an annoyingly dim but still useful rf patch.
in the worst case scenario it's possible your repairman swapped out your viewfinder mechanism for another one he had laying around....but that is just paranoid talk and pretty unlikely especially if you trust your repair person.
Epson R-D1, Ricoh GRD III, Rollei TLR's, Lots of others