Hi Otheos; Has your issue been resolved..?
I was experiencing that similar strange issue after upgrading my BIOS as well... In addition to what BigJohnny said, I would start the system to windows (if already installed) with the default settings... I had to to clear the CMOS several times, as part of the RAM training.
Once you've verified it works fine, boot back to BIOS and enable the first XMP level (leave the voltage as auto); boot back into Windows...
Once that is fine, do it again with XMP II enabled...
At least me ROG Z390-E seems to test and re-train my RAM every time it start. I have it running with default voltages, except for the CPU...
My build...:
ASUS ROG Z390-E, BIOS 1401
Intel i9-9900K, 5GHz 1.375v, w/ 4 AVX Offset and level 7 for Load Line 😄
HyperX 32 GB (8x4), 2666 MHZ, 1.2V, CL16