When forming a group for the arena, it subtly removes your real group, should you and your arena teammate have been grouped. There is only a "X no longer follows you" echo.

Then, in arena waiting room, should your transmuter teammate try to haste you not realizing your group ended, you are trapped and one of you can die very easily with no escape.

I think that dying in the waiting room should count as an arena death, not a real death. ...not you've trapped yourselves in a room of real death with real consequences.

or

The arena waiting room should protect teammates from attacking each other (though, that probably is more involved).

or

The arena should not ungroup grouped characters when they make an arena team.