What you were expecting to happen, and what actually happened:
Neither Diamantina, nor Stellarix, is of fey type. They should not appear here in Winter’s Reach (Fey).
How often does this happen? When did it begin happening?
Today, I think?
Steps to make it happen again
Fight some PvP battles in Winter’s Reach, you should see it sooner or later.
This can happen in a very small window after weekly reset while the region restriction is changing and while players are setting up their new PVP team that matches the new weekly region restriction.
You shouldn’t have any problems with it by now, if you’re still seeing teams like this though, please try exiting and re-opening the game to ensure your game client is pulling from the latest PVP data on the game server.