The Samsung 980 NVMe is correctly detected by the second LP 3 I just received. So my first one is definitely faulty, I'm gonna contact the support to arrange a replacement.
I tried with a Samsung 970 EVO, same issue.
I will have access to another LP 3 next week.
I see two options, the first one of my LP 3 is faulty, I will be able to verify this once I get access to the other one. The second option is both Samsung 980 and 970 EVO aren't compatible with PCIe 3.0 x2 (not x4), which sounds unlikely to me.