What programs are you running when you hit the 16GB limit?
In other words, what is in the "Processes" tab?
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It appears you have 2 x 16GB chips for a total of 32GB.
You have 64bit Windows, so it can handle 32GB of RAM.
But Windows is peaking at 11.1GB.
Very strange
It looks like you are hitting some sort of software limit, since it does not appear to be a hardware issue.
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Try going crazy. Open a bazillion tabs in your browser. Open youtube in each tab. That should push your RAM, and CPU usage up.
The Chrome browser has a reputation for using lots of RAM. FireFox is one of the biggest users on my system.
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Do you have any other programs that do a lot of processing? Do you do any video processing? Or photo manipulation? Load several 4K images into GIMP (it's free). Load some 4K videos into VLC video player.