Best Friends Trading Issue: 3-Star Pokémon Turning into 2-Star After Trade - What's Going On?

This is the first time that I've encountered this issue however. I'm "best friends" states with a person I'm trading with and they gave me a three-star Combee but when we traded the Combee changed to a two-star. So we tried another Pokémon that were both three stars and it did the same thing and switched it to two stars with less stats. Why is it doing this all of a sudden?