Ah I see that’s not the transformer now… it’s probably built into the RJ45 connector. The twisted pairs look to be in that black bundle that runs down to the RJ45 on it’s own little board with a weird bundle termination… Still, lots of places for a broken or bad connection between there and the Broadcom chip at the other end of the braided cable.

Is the 16-pin chip on the underside the ethernet line side transformer?

If so can you check out the connections? Your photo shows 2 of them looking very dodgy.

My iMac is not able to get more than 100M through that connector and I’m guessing one of these connections is bad on my power brick also. Apple might have a quality issue here (though many wouldn’t notice the speed drop even if they use this niche port).