While having the same issue with Micron/SK Hynix & Samsung I figured it's not about 2133, 2400 and/or 2666Mhz or the vendor. My SM boards just do not like 4DRx4 ram. Every 2Rx4 ram, no matter of the vendor and/or frequency just works fine. So don't let someone mislead you that EPYC's do not work with 2133Mhz. They work perfectly fine. My H11SSL-i and H12SSL-i just don't like 4DRx4 while my Gigabyte MZ32-AR0 just doesn't care as long as I use max 8 sticks only. So it has something to do with the 4DRx4 typology.
note what AMD writes:
2 DIMMs Per Channel 2666 LRDIMM operating at 2133 MHz
the difference between RDIMM and LRDIMM is the LRDIMM contains additional buffers for all data lines.
this requires additional clocks to control. a 4Rx4 LRDIMM is 4 1Rx4 DIMMs in once with the memory-controller signal load of only one DIMM.