I used to resist the whole "Su is working with the Red Lotus" idea at first but when I rewatched last episode I noticed a few things.
This is Aiwei when he was going to question Su
His eyes are worrying me. The way he moves them. Seems like we has thinking: "Oh, shit, they are going to find out. No wait, I can lie and tell them she is saying the truth".
And this is Su after the Krew revealed that Aiwei was the traitor.
Those fucking eyes, again. She’s planning her act.
This may sound stupid to you but we all know how Bryke likes to do this kind of stuff. They both seem pretty suspicious. Like, remember in Atla when Ozai’s Angels disguise herselves as the Kyoshi Warriors and Azula says to the king:
"It’s terrible when you can’t trust the people who are closest to you."
For a really tiny moment, you can see Ty Lee and Mai looking at eachother, foreshadowing their betrayment in ‘The Boiling Rock’.