Yes, this is happening to me as well.
I was winning 5-2 in FUT Champions and my opponent left the match near the end. Instead of getting the win, I got a loss. It’s honestly very frustrating and unfair.
My connection is stable (I use Ethernet too), and I don’t have issues in other matches, so it doesn’t seem like an internet problem on my side.
I’ve seen other people mention this before, so it might be some kind of glitch or exploit. Either way, it really ruins the competitive aspect of the game.
I don’t think there’s a real solution right now, but I’d recommend reporting the player and contacting EA support, even though they usually don’t refund coins or give the win back.