"Once and for all: there is no boost applied to the defending AI team in the arena, as far as any person working on the game is aware. I went and had a chat with the engineers who built our most recent improvements to the arena system just now to confirm this. The AI used by the defending arena team is identical to the AI used by your team on autoplay."

Did you ever think about the possibility, that there could be a bug involved (ok, I know, LotB is almost bug free and everything is working like it should). But there were once relics, that should only work in gauntlet, but also worked in arena. Maybe that gauntlet advantage crawled silently and secretly to the arena?