Question Only half of the memory is available

[SignumPL]

Hello! I discovered a very strange issue on my nephew's computer.
He has 16GB of RAM (2x8GB), and Windows 10 detects all the memory but shows only 7.85GB is available. I went to BIOS and it shows 16GB is physically installed. I ran memtest86, and it also shows only ~8GB is available, same as in Windows, however there were no errors. Then I removed one of the modules, and tested it with memtest86 - no errors. Then I switched the modules, and tested the other one - same result, no errors. XMP is disabled. I don't have idea why both modules together doesn't have full capacity? What else can I check?
Everything was fine at the beginning when the computer was new, but at some point (hard to tell when exactly) it started to not using all available memory.

OS: Windows 10 21H2
Motherboard: Gigabyte B450M DS3H
RAM: Corsair Vengeance LPX, DDR4, 2x8 GB, 2400MHz, CL16 (CMK16GX4M2A2400C16)
CPU: AMD Ryzen 5 2600

 

[Lutfij]

Welcome to the forums, newcomer!

What BIOS version are you currently on for your motherboard? You could try and reseat the sticks of ram sequentially and boot the system with each stick of ram. I've had one system behave the same way. Until I powered down, took all the sticks out(4x4GB kit), then cleaned the gold contact points on the sticks of ram(using an eraser), reseated them in one slot first. Powered up the system, showed the stick of ram both in BIOS and OS GUI, then powered down and followed up the same manner with the other sticks, hence why I suggested sequentially reseating them.