when you rooted your phone, there's a modified inside the system, that's why samsung bootloader will detect it as "modified/custom" instead of "official". I know your ROM is still official after you unroot it. But, unroot just remove the access to the root/system file. Whatever modified during the rooting process earlier will remain the same. Therefore, if you want to get an "official" status back, you have to flash stock original firmware after you have clean binary, & unroot your device.