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The problem may be that bp_message_thread_has_unread() is hard-coded to check based on the loggedin_user.
See bp-messages\classes\class-bp_messages-thread.php

If you remove your $unread conditional, you will see all the messages for $UID.

To only get unread messages, you may need to use custom sql that starts with the bp_messages_recipients table and queries based on the unread_count field.