A few quick questions...
Have you got any other features within the mod running, for example the User Threads part which allows the post starter to edit the other posts in their thread?
Have you got any other mods that also adjust the moderator abilities or editing abilities of users?
Can you provide me any more details of the usergroup and forum settings for that particular thread and the forum it is located in?
If you want me to take a look as I get time this weekend (I'm away from home for a couple of days), feel free to PM me a user account and password on your site. If you do want to do that, admin permissions that allow me to see the usergroups, forums and Threadmins sections would be important.
I'm currently trying to track a couple of bugs within Threadmins, so this may well help me identify some of the issues we have.
Thanks for posting the problem.