Memory speed troubles
Memory speed troubles
posting and booting into an OS are different things. For Ryzen, the procedure is: AGESA initialize --> BIOS loads --> Boots OS from boot drive. It could very well crash before it managed to boot, so it rolls back to previous phases with the last stable setting (or whatever the SPD profile has that's JEDEC spec) and manages to boot.
You haven't evaluated enough memory kits. This seems accurate since most units rated over 2666 typically feature a 2133MHz CL15 profile as their second fastest speed in the SPD chip, which is what manufacturers aim for. If a kit offers a 2666MHz CL19 profile at the same time (SPD chips generally support up to four profiles, including the exclusive XMP one), the board would attempt 2666MHz CL19 first.