As I said, Ubuntu has zero issues with handing 28GB+ RAM to applications, so this is convincingly a Windows thing and not something related to the BIOS.
While searching for a solution there's another person having this issue with a Gigabyte Z97-D3H-CF mainboard. He has 32GB RAM and half of it goes to "Shared Video Memory". I downloaded the manual and the relevant section reads
`Intel Processor Graphics Memory Allocation: Allows you to set the onboard graphics memory size. Options are: 32M~1024M. (Default: 64M)`
So the mainboard has no option for 16GB "Shared Video Memory", yet Windows 10 is causing this problem to him [0].
For me the "Works in Ubuntu" is what makes me disbelieve the BIOS-settings claims.
[0] https://answers.microsoft.com/en-us/windows/forum/all/change...