Same Problem here. Bought a ASUS TUF FX505DV with AMD Ryzen 3750H with RTX2060, it has 2x8GB DDR4 Micron 2666Mhz installed and CPU-z shows FSB:DRAM async.
Dont know why, may ASUS normally build Intel systems so they don't specifically buy 2400Mhz RAM for AMD setups.
But as you can read on Wikipedia front side bus you see an entry "overall system performance can vary in unexpected ways with different FSB-to-memory ratios".
So i decidet for my personal solution to buy a new pair of RAM, now that they are so cheap anyway, 2x16GB 2400Mhz to bring the FSB:DRAM Ratio synchron.
May there are other solutions with BIOS settings?
Hope i could help you a bit.
By the way, thanks to AMD, the Ryzen 7 have good performance. I am positively surprised!