This was on a client's system, so I don't know what led to the problem in the first place, but he brought it to me with the symptom of "it keeps turning on and off".

Windows 10 21H2
Gigabyte X390 UD motherboard, Intel i7 9700K processor
Corsair Vengeance DDR4-3000 MHz 2x8 (on Gigabyte's QVL for selected CPU)
Nvidia GeForce RTX 2070 Super (I think)
EVGA 850W PSU

Hard drive structure was a mess; the boot drive (m.2) was MBR (could boot with legacy boot selected and selected in BIOS) and the only GPT drive was a 1 TB spinner with no Windows folder.
Booted into MediCat (Ventoy edition) PE disk and made all drives GPT. Shrunk boot drive enough for EFI and MSR creation; deleted the unusable EFI and MSR partitions on the 1 TB spinner. Expanded another 3 TB drive to full size.
Booted back into MediCat and shifted to EasyUEFI boot disk. Recreated EFI partition contents and rebooted into m.2 drive with GPT boot. All is normal that way.

Hardware Reserved Memory problem showed itself when I booted: 14 GB of 16 was reserved by BIOS and unusable to Windows. I had already set BIOS to Optimized Defaults, and there was nothing to change that helped, not even (Under/Above 4G). Placing the RAM in the other two slots didn't help; one stick at a time didn't help. New RAM didn't help. Registry Fix to clear pagefile didn't help. bcdedit commands didn't help; to get there I had to recreate his recovery partition on the m.2 drive. Flailed around a bit after that until I noticed that when in MediCat, I could use and see all 16 GB of RAM available.

EDIT: There was evidently something wrong with his Windows installation.

Asked around and learned that Windows Setup goes into PE mode after initial GUI setup is done and the first blue screen appears starting SAFEOS phase of repair install. Chanced the repair install and it worked! All of his RAM is available now. Everything on the web never said that a repair install might fix this problem.