I've already upgraded the NVM firmware on the Intel card to the latest, no change in behavior
Are you sure about that? Your dmesg says differently:
ixl0 at pci1 dev 0 function 0 "Intel X710 SFP+" rev 0x02: port 0, FW 6.0.48442 API 1.7, msix, 8 queues, address 40:a6:b7:b3:4b:28
In contrast my dmegs outputs: ixl0 at pci1 dev 0 function 0 "Intel X710 SFP+" rev 0x02: port 0, FW 9.152.77998 API 1.15, msix, 4 queues, address 40:a6:b7:b5:4a:c4 According to Intel, your firmware is more than 6 years old: https://www.intel.com/content/www/us/en/download/18639/28335/non-volatile-memory-nvm-update-utility-for-intel-ethernet-adapters-700-series-efi.html That's to not suggest that's the reason mind you. I've been issue-free with my Intel X710-DA2 using a customized 10G SFP+ passive direct attach copper twinax cable from FS with the other end connected to a Juniper EX2300-24MP switch.
