Here is my bug report:
https://connect.microsoft.com/SQLServer/feedback/details/3022019 and it is still in active.
It happen one of my production database, when I tried to remove the database from alwaysON and other important databases went recovery pending and inaccessible. I have no idea what happened to all other databases, then I came to know it is a bug.
The issue is when the database removed from the primary replica, with the secondary disconnection the higher database IDs on the secondary went into “NOT SYNCHRONIZED and RECOVERY PENDING” state, but the lower database IDs are good and synchronous state only.