I have just updated to Beta 7 including updating Flagrow byōbu. Everything went fine, private posts are properly hidden when the extension is disabled.
One reported bug is still present though:
Apoculos Only Admins are allowed to edit user participants once a private discussion is started... regardless of how I setup the permissions.
This brought me into a strange situation at my local testbed installation: As an admin I initiated a private discussion with another user and before sending it (or after the fact, I don't remember exactly) I withdrew myself from the recipient list. After that nobody was able to correct this mishap. As an admin I could not open this discussion (even though I started it), and the other user could not add me to the recipients again.
The bug that I Pollux earlier is now halfway resolved. Private discussions in the All Discussions view get their byōbu-decoration and the recipients listed even without having visited such a discussion before. The same however is not true when I enter any tag view with private discussions and reload the page. As before there will be no more visual indicators for a private discussion. To make them visible again, I need to look into said private discussions (as reported before) or reload the All Discussions view (new).