PCIe link speed negotiation can be tricky: assuming both the slot and the adapter support PCIe 4.0, auto negotiation can still get it wrong if there are firmware bugs or signal integrity issues. Not on P620, but I’ve seen a system which had two Mellanox ConnectX-5 NICs (the PCIe 4.0 model) on different PCIe risers, one would negotiate PCIe 4.0, the other 3.0…
First thing I’d make sure is to have latest BIOS on the P620 and firmware on the adapter and see if that gets auto negotiation to work, it’s usually preferable if auto negotiation just works, but sometimes you do end up having to force the speed.