linus,
I'm not sure what's occurring to cause the issue you're seeing, and it's not something we've seen or heard of before. Just to make sure, you are using Joomla 1.5, correct? The force password change plugin hasn't been tested with 1.6 at all.
Other than that, our plugin does not in any way modify the jos_core_acl_groups_aro_map table. The only table we modify is the jos_users table, and that's to set the users lastvisitDate to make sure that they are properly directed to the edit user form.
If the aro_map table is being modified, my guess is that it's by another plugin on your site. Do you have other ACL, user manager, or anything related extensions that may be trying to modify the user? There could easily be a conflict with something else, but I just don't know what it would be offhand. Please let us know any other extensions your using that may be related, or try unpublishing them to see if that fixes the problem.
Thanks!