I think the game is now preset to R6 and won't allow R15 anymore. I've spent the last 5-10 minutes rejoining with different avatars and keep ending up in only R6, unless there's a different game or a way around this I'm pretty sure it's only R6 now.

Add: It was updated 6/29/25. That may have been when the change was added as the most recent run from 2 months ago, still uses R15.