I apologise for the thread necro, however I was having the same issue that eribob experienced but with only 64-GiB of DDR3 ECC showing instead of 128GiB. With the lead from eribob's digging, I found the exact same problem with dmidecode and discovered there is a typo in the DMI Table (Instead of 0x17 (23 bytes) length being used, it was 0x23). After a bit of researching and work, I've now modded my bios (no updates from Huananzhi were found) to fix this. Unraid 6.9.2 now shows all RAM now.
I have a rough guide for others who experienced this (from my research, it looks like it's any chinese dual socket 'x79' board is affected), to fix it as well here: https://www.win-raid.com/t9047f16-Request-for-help-fixing-DMI-Tables-on-a-Huananzhi-X-D.html
I also apologise if the link breaks any rules, I don't see anything that prevents it, but I might have missed something.