It really seems to be a bug in IE, you are allowed to set the parent of a forum to itself, effectively hiding it from view. The only way to get it back it to change the parent id directly in the database.

You are allowed to do this because IE ignores the disabled attribute inside the drop down options. It grays the option out but you are still allowed to select and use the option. It works correctly in FF.

Wouldn't a better solution be to remove the direct child forums from the drop down instead of disabling them?