I really hope I'm not going to offend anyone with this reply ...
Sometimes it's beneficial to have 2 tools which basically do the same job, i.e. Sigil and calibre Editor, because both have some features (and problems
) the other doesn't have. This might be a case in point.
If an image is present in an epub but is unmanifested, the calibre Editor will display it in the images file section marked as unmanifested.
I'm making no subjective statement about which is "the best epub editor" just that you can't go far wrong if you use each tool for what it does well