Hello

1. A non registered user create a new thread.
2. A mod edit this posting. --> "edited by mod" is shown
3. A mod changed the category --> "edited by non registered user" is shown

Until here I can confirm your observation. I create a thread as an unregistered user in one browser and alter the content of the entry as the admin in another browser. After this first edit the username of my admin-account is shown, if it's not disabled to show the admins name. After changing the category as the administrator in a second attempt, the name of the (unregistered) thread opener is shown in the entry-is-edited-message.

I have the forum set up so Guest's can post but not edit their post, so if a Guest post is edited or changed by a Moderator it should never ever show as "edited by Guest" but it does if you follow this chain of events that I posted.

1. A guest creates a new thread.
2. A mod changes the category AND edits the posting --> "edited by mod" is shown which is fine but
3. A mod changes only the category again --> "edited by guest" is shown