After an update to 3.6, the changed privacy settings in messages apparently allow only messages from "contacts". This means that after the deadline, all messages received before the update do not longer show up in the message interface. Note that the messages are not actually "deleted" and can be accessed if one remembers the "chat partners". However, this is rather awkward and relies to much on personal memory.

To reproduce:

1. Exchange a couple of messages between users in Moodle prior to version 3.6. Check that your messages window contains several messages from different users.
2. Upgrade to 3.6.
3. Access your messages: there are no messages, even if you go to "Message settings" and allow messages from "anybody in the system".
4. Search for one of the contacts that sent you a message (or that you sent a message to) in the 3.6 message search.
5. Click on the (correct) user and all previous messages will be shown. However, to access them again, you will either have to repeat steps 4+5 (search and click), or add that user as a contact - which requires them to confirm this, so it may take a while (or may never happen).

At first glance (and not only that), it appears that all messages were "deleted", therefore I am flagging this as a bug. It would be good to at least have an option to see the list of users - somewhere, somehow - that I exchanged messages with, even if this only concerned messages in the last X days (for privacy reasons).

See also the "General help" Is it me or is messaging in 3.6 completely dysfunctional? forum thread.