Typical symptom for me now is that packets flow only in a single direction over this interface. Sending from v1.2a does not work at all, but receiving do. So, v1.2a fails to lookup ARP for example, but receives network noise just fine. See my inquiries about that in past: V1.2A: 1G network jack not working, 100M network jack does and github issue link in that post aswell, where I did a test report about this.
Note I did not try to fix it by updating to current dts from git. I do run quite old rebased LTS kernel and I don’t run Debian and I currently don’t need 1G port now (I just wait for official kernel upstreaming process to complete, without even GPU opensource drivers, eMMC performance fix, NVMe fixes there is little to see currently. I enjoy hacking it instead). I know SF tried to “livepatch” dtb in memory directly from U-Boot, but it never helped me anyway.
Don’t get me wrong, I don’t want to persuade you to keep potentially faulty hardware nor trying to defend SF’s image there. Just my prior experience. If you try to observe this with e.g. tcpdump and see same results as I described then probably your board is fine but software is messing things up. Again, U-Boot AFAIK shall always work.
AFAIK v1.3b (most current revision) is unaffected by this at all and shall work with two ports fine. They use improved chip, related to one v1.2a got at driver level but better version, not the same 1G found in v1.2a.