Something similar happened to me with regular Facebook messages. Facebook, to combat spam, decided to introduce the concept of folders for messages. Besides the regular folder for messages from friends, there was ”Others“ into which all other messages were put.

The UI didn’t make it very prominent and I learned about the Others folder only when I read an article about how someone missed appointments for job applications, invitations and messages from acquaintances because Facebook sorted these messages into the Others folder. Back then this happened quietly, without notification of the user. I checked my Others folder and found years old messages. Insane. All the missed opportunities and trouble that was caused for millions of people because Facebook intervened in their communication.

I think Facebook now at least informs the user if they received a message from a non-friended person. Took them a long time.