Update: It looks like Xbox Preview owners will have their Xbox 360 games back up and running on the Xbox One soon.
Backward Compatibility titles will show up in Collection for Preview members hopefully in next ~2 hours or so. Thanks for helping us test!— Mike Ybarra (@XboxQwik) June 24, 2016
Microsoft says it is working to fix a bug in the current Xbox Preview build for the Xbox One console that's preventing users from accessing their Xbox 360 backwards compatible games.
Microsoft Xbox team member Bill Stillwell mentioned the issues on his Twitter account:
Xbox Preview customer and having BC game issues? Don't uninstall and try to reinstall! It is not your issue - we are looking into it.— Bill Stillwell (@wcstillwell) June 24, 2016
There are now over 210 Xbox 360 games that are backwards compatible with the Xbox One, including eight games that were added to the list on Thursday.