I have been to about 15 different trial chambers in the preview. Not a single one had this new encounter room. Don't know if its a bug or no but either way it really doesn't generate enough for such a cool feature. So i'm still yet to see this new room in-game. I dont think there is enough room for it to generate in corridors with all the other rooms jammed in around the chamber.
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We're tracking this issue in MCPE-184840, so this ticket is being resolved and linked as a duplicate.
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