Make sure you have the latest bios. The B550 came out before ReBAR was announced I believe. It was added to my Asus bios in the Feb 20. 2021 update, wasn't there in the Sept 2020 bios.
Components matter. AMD-Radeon gets best results currently, in more areas, the Intel-nvidia is touchy, it can actually lower performance by 1-2fps or tie in most games. Only few titles take advantage of it, about 10ish fps gains. I expect it's similar to Raytracing/DLSS in the 2000 series boards, it'll get improved in the next gen stuff, being just the first iteration for Intel-nvidia.
Not sure how well it'll work cross-platform (Intel-Radeon / AMD-nvidia)