10.3.2.2 Handling of GATT indications and notifications

A client requests a server to send indications and notifications by appropriately configuring the server via a Client Characteristic Configuration Descriptor.

Since the configuration is persistent across a disconnection and re-connection, the client shall check the security requirements against the configuration upon a re-connection before processing any indications or notifications from the server. Any indications or notifications received before the security requirements are met shall be ignored. Any indications received before the security requirements are met shall be confirmed and then discarded. When a client reconnects to a server and expects to receive indications or notifications for which security is required, the client shall enable encryption with the server.

If the server does not have an LTK, indicating that the server has lost the bond, enabling encryption will fail.

See Errata here: https://www.bluetooth.org/tse/errata_view.cfm?errata_id=18349