Recently, Windows Insiders reported that a bug has caused their devices to enrol into the Release Preview Ring even though they had selected Fast or Slow Ring. If you were one of those who noticed this issue then don’t worry about it. Microsoft recently acknowledged that it was indeed a bug and confirmed that it has been fixed.
The issue was reported on Reddit where users confirmed that their devices enrolled into Release Preview automatically and there was no way to revert it back. Now, Windows insider Engineering Program Manager, Eddie has posted on Microsoft Answers forum confirming that the issue has been fixed.
Hello Windows insiders! Some of you may have discovered today that your Windows Insider ring settings may have changed from either the Slow or Fast ring over to Release Preview and you can’t change them back. We’ve identified and resolved the issue that caused this, and your devices should return to their Previous ring next time you sync.
As noted above, you should be able to switch back to Fast or Slow Ring once you sync your device.