This is happening to me too.  I have 15 forums, 5 public, 5 restricted to 1 group, 5 restricted to the other group.  I dont have any linked forums and need a way to fix this, any ideas?

I didnt see any errors, it just seems to be a rendering issue.  What i did to get this was make a page that links to a forum page (the forum page wasn't a child of any page and was hidden from the menu).  From there, I created 3 groups that had permissions on the forums, first was Registered Users, Second was CustomGroupA and Third was CustomGroup.

I gave Registered users permissions to the first 5 forums in the first group.  CustomGroupA and CustomGroupB got permissions to the second group, and CustomGroupB got permissions to the third group.  It worked fine while setting it up, it wasnt until I tested the site and logged in with a RegisteredUser group acct, saw that it was only showing the first group as expected.  I then logged in with the Host account to change a few things and viola, the last 2 groups were missing, and not matter what i did, i couldnt get them back unless i deleted the forum and removed from recycle to another page.

This is still happening on my site, I am more than willing to give you the log in info so you can see it for yourself. Just send me a PM and i will set it backup so you can see it.

Thanks,
Dave