I've just tested this out on one of my own sites, and the Manager role is able to change their subscription settings on an individual thread, or across a whole forum, where the forum is in optional or auto-subscription mode. Obviously, if subscription is disabled, no one can subscribe. If subscription is forced, it should not impact the manager role, as by default the capability for allowing a user to be force subscribed is "Not set" for the manager role - see here: https://docs.moodle.org/28/en/Capabilities/mod/forum:allowforcesubscribe
My suggestion would be check the subscription setting on the forum, and the permissions of the manager role on your site. You'll want the "Manage subscriptions" capability set to "Allow". Remember too that permissions for roles can be adjusted at site, course category, and individual activity level.
Hope this helps!
All the best,