Exactly same problem with HERO XI BIOS 0805, and HyperX Predator 3200Mhz 32GB (8x4) HX432C16PB3K4/32 (Hynix)
Sometimes the system may not start. I tried different frequencies and voltages - full random. The system can start ten times in a row, but not start the 11th time! And it will not start until you reset the BIOS settings or enable MemOK. At the same time, I calmly pass the 24 hour stress memory test if I can boot!
Please help!! On my old z370 g-gaming everything was fine for 6 months!!