I just noticed that MP3tag is using my data around 20GB in 30 days. Is MP3tag uploading all my Mp3s which I edit somewhere ?
If not why MP3tag should use that much data ?
(Sure we all got a copy of your files 
- No, just kidding!)
Where are your media files located? Somewhere in the local network?
"Ethernet" is internet traffic but local (LAN) traffic too.
I guess you did edit about 20 GB Files with mp3tag from another device in local network,
another computer or nas.
regards
Peter
( Where should i send my DMCA Notice ? 
)
Yes the files are stored in a network share. I didn't know Windows 10 includes LAN traffic in Data Usage.
Windows can't know where LAN (maybe complex one with different SubNet's) end's up and internet begins.
It's up to your network - at home normaly just an simple internet router - to route to the internet if destination is outside your LAN.
For Windows it's all the same, goes out/comes in over your Ethernet Card.
Btw. if you would still use an simple 56k Modem, you would have an statistics for internet (DFÜ) 
,
same if you would use an VPN, maybe to connect to your office.