Not a bug in fact you probably where using a way older version because the admin edit categories has been like this for along time. If you have over 30 sub categories the subcategories are not shown only the top level categories and you will see the top level category is clickable. If you want to edit that top level categories subcats simply click the hyerlink to see the subcats. This was added way back in the PhotoPost 6.x series if I remember correct. If you never saw this because and where not using older code then you most likely recently added some more subcategories to trip the code that surpresses the subcats.