That's exactly the same problem i have on my 5800x. I was trying to figure out whether it's a specific issue on Gigabyte BIOSes or maybe an AGESA 22.214.171.124 'fix' to allow >1900 IF clocks.I updated to the AMD AM4 AGESA V2 PI 126.96.36.199 Beta BIOS(5833) for my ROG Strix x470 board but it seemed to lower scores by 2-3% and PBO for my 5800X was locked to 5025mhz boost. Where before I could clock as high as CPU would let me(5100mhz) under old BIOS(5809). Went back to old BIOS for now till they post non beta version.
(i could never even post beyond 1867 on 188.8.131.52, on 184.108.40.206 i can get to 2000 although i'm not happy with the RAM and SOC voltage requirements, so i'm currently at 1966 stable)
I guess your comment does at least answer my first theory (it's not Gigabyte specific...) however i really hope it's not meant to stay like this in the final version, cause it's just extra performance down the toilet...