Arena just creates a QM game except as if you had only that Arena template enabled in QM. It sounds like this is a bug that should be reported, since Arena games should also set your coin wager to 0 on top of that.
Go to: https://warzone.com/ReportBug
and file the report saying Arena games have unexpected behavior of creating coin games. Hopefully this is not the intended design, because (in effect) forcing people into coin games they didn't mean to sign up for is really not nice.
Edited 12/23/2021 18:32:38