Found this info in this link https://forums.tomshardware.com/threads/windows-10-only-using-half-of-installed-ram.2558441/
Also saw where this was a bios issue on a Gigabyte H170 board, Do you have the latest Bios?
Also you could run Memtest86 Memtest 86 http://www.memtest86.com/technical.htm
Guide to using Memtest86+
https://www.wikihow.com/Test-PC-RAM-with-MemTest86 This runs outside of windows and if it shows only 8GB it is a motherboard issue.
Did you mean this happened after you install the latest bios file?
Already tested all bios versions!Maybe you need to go back to the previous bios. What Bios do you have now?? Some times they put out Beta Bios that is not ready for prime time!