A little update: tried both sticks in my friend's computer. This guy has a B450 Asus board: the good stick worked as inteded, the bad stick didn't even make the computer post at startup!
Then I had the chance to try my ram in another friend's computer, this time a Gigabyte X470 board (like mine, even if not the same exact model) and the behavior was very similar to the one I have on my computer. The PC starts and gets to Windows with the faulty 8 GB stick but doing a memtest86 causes the pc to reboot.
The only thing that still does not convince me is the fact the sometimes it takes a little bit longer for the memtest to fail... sometimes it fails very quickly, other times it takes a minute or so. The answer I gave to myself it's that probably there is some "sector" of the ram that is damaged, and when that is accessed, the freeze occurs.
I have to go through the RMA process now and really don't know how much time it will take.... so in the meantime I bought some 4000 MHz Kingston (HX440C19PB3K2_16). I know they won't ever go to that speed but they're B-die so they should be a little better at least.