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I'm not too sure what you mean by third person/mixed reality mode being turned on - as far as I can remember, I haven't added anything like that into the game (unless it's something built-in to the SteamVR plugin..?)
I can't really help out with that unfortunately; I've got no way to properly test that sort of functionality, so I can't find out what could be causing the problem. Sorry about that
If it's a built-in feature, then it's probably interfering with the UI pointer system being used - if you can point the camera/view at a menu, does a pointer show up?
If it's doing what I think it's doing, I might be able to fix it pretty quickly; I'll take a look at the code and see if I can get something working in the mean time.