A one-fix patch.
Admins deliberately cannot leave channels, so whoever runs the server can see what happens in all of them. Transferring admin quietly broke that: the incoming admin only had the channels they had already joined as an ordinary member, and since admins cannot join or leave channels, no way to pick up the rest.
They are now added to every public channel as part of the transfer. Existing memberships are left alone. Hiding a channel from your own sidebar is a personal view setting and still works, which is what makes the no-leaving rule liveable.