Question Super weird ram problem

[maverickx029]

My friend bought a used computer. a8-7650k, dual Team Elite 4gb 1600 ddr3 ram, shitty 150GB HDD and a Biostar hi-fi a70u3p (windows 10 x64 1903).
The problem is, one of doesnt work properly. I tested ram individually and heres what happened.

stick A in ram slot A & B and they work fine .
stick B doesn't boot on both ram slots.

Here's the weird part. When I installed both RAM, the pc boots up and runs normally and shows 8GB ram total but only 3.42GB are usable.
We can assume that the reason it is only 3.42GB instead of 4GB because 512MB is allocated for the iGPU right?
The thing that I don't understand is why did the PC boot up normally when used together with the working ram?
normally, the PC won't boot when one stick is faulty even if one of them works properly?
Of course, I told my friend to bring it back from the seller and change the ram or just refund the pc.

edit: added windows version

 

[Satan-IR]

Are both RAM the same, from a package, or different modules? Just to make sure there are two of 4GB making up 8GB?

What is the OS? if a 32-bit Windows one will shows around 4GB.

 

[Calvin7]

Supported memory for the A8-7650K is 2133MHz.

dual Team Elite 4gb 1600 ddr3 ram

Memory modules sold as Singles have no guarantee to be compatible if used in multiples. If you have a question about memory, I suggest you list a part number or link to it.

 

[maverickx029]

Are both RAM the same, from a package, or different modules? Just to make sure there are two of 4GB making up 8GB?

What is the OS? if a 32-bit Windows one will shows around 4GB.

q1: i dont really know. As i said, he bought it used. Im not sure if its a dual channel kit or single
q2: yes two 4gb stick that should be 8gb in total/
q3: it is 64bit, i just installed windows 10 home x64.

 

[maverickx029]

Supported memory for the A8-7650K is 2133MHz.

dual Team Elite 4gb 1600 ddr3 ram

Memory modules sold as Singles have no guarantee to be compatible if used in multiples. If you have a question about memory, I suggest you list a part number or link to it.

I think your missing the point, even when using it as a single stick, it won't boot on its own. The weird part is it will only work if its paired with a working one and will only be detected, not added up.