SFP and cable are good, but don't show connection. Loopback plug does somehow?

Hi,

I am currently troubleshooting an Arista 7050SX3-48YC8.

I do not have write privileges to the switch, or I could troubleshoot from there inside the switch.

We have tested the SFP in other switches, tested the fiber cable, tested the light levels. None of it is faulty or broken, and we replaced it anyway.

Somehow a loopback plug works.

What does this mean? My first thought is its a configuration issue and the network engineer needs to fix it, but I have no idea.

This is both for my own information and learning as well as help.

Thank you!