Thanks for your help guys.
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.
These are Kingston too, let's hope for the best-
Gigabyte X470 AORUS GAMING 7 WiFi
ROG STRIX RTX 2080TI OC
2×8 GB DDR4 Kingston HyperX Predator 4000 MHz B-die
Samsung 850 Evo 250 Gb
Western Digital Caviar Blue 1 TB
EVGA SuperNOVA 650 P2
Thermaltake Core X5 TG
ASUS ROG Swift PG279Q
Corsair K65 Rapidfire
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