I would try a different benchmark, I think Cinbench r20 uses instructions that could prevent a higher boost. I also think that top single core boost may only show itself under lower load, like a game. Maybe try r15 and/or a GPU limited game. Or even just try video playback in a media player.
Unfortunately you kinda need all the stars to align to get that top single core boost to show up. But I've seen it in reviews for games, pretty sure Far Cry 5 bench can do it for example.
Cyberdyne_ got a reaction from Origami Cactus in Cannot get 3900x to Max Boost to 4.6Mhz.
