100% agree with BabakOnly and have replicated the issue.
I bought three 4K AppleTVs and their iOS version out of the box was 11.1. I configured the first AppleTV 4K how I wanted it, was attached to Ethernet just fine and as my last step, I ran the updater with took the AppleTV to 11.2.5. After that, it would ONLY see wifi. I confirmed the port was still active, still assigning an IP, etc. No other change was made other than the OS update. I reset this AppleTV, however it didn't revert to the 11.1 it shipped with, it still was at 11.2.5. With no USB ports, I don't see how I can get this device back to a stable version. Our wireless signal in our office is HORRIBLE and I must have it hardwired.
I put that AppleTV 4K aside and replaced it with a new boxed one with 11.1 installed. Again, when through same process of configuring it, same port used and it pulled from ethernet. I did NOT update this one to 11.2.5 and it's working fine via ethernet. I will be taking the one that's at 11.2.5 to a Apple Genius Bar to see if they can revert the software on it so I can hard wire it. If not, I'll have to do a return/repurchase to get a device that isn't at 11.2.5.
In short, 11.2.5 does break ethernet connectivity. If you need the hard wire, do NOT upgrade until Apple fixes.