Hello,
1. Not all CPU can achieve 1900FCLK. These things become harder to achieve with 4 modules, single or dual rank. The fact 1933 and 1966 can POST and 1900 can't, but not stable could be a number of things attributed to signal margins/auto rules for some CPU and/or CPU limitation
2. Does the issue occur when the system as at Optimised Defaults (i.e no overclock of any kind, inc XMP).
Thanks.
13900KS / 8000 CAS36 / ROG APEX Z790 / ROG TUF RTX 4090