We're investigating an issue where users are not able to subscribe to bots correctly, with some of them shown as Unknown in the Audience tab with 'Deleted Conversation' status, and others not being displayed in the Audience at all.
This looks like an issue on Facebook's side - a public report is already created, and you can track this issue's status here: https://developers.facebook.com/support/bugs/484143269217803/
More information to follow soon.