Having read through the discussions, there would appear to be a simple error.
While it is true that both erect_nipple and covered_nipple both suggest the existence of a 'nipple', the reverse is definitely not true.
For example: 'erection' implies 'penis'. But just because there is a penis, that does not imply 'erection'. This would not be a case where alias could be used, as alias means the implication applies both ways.
For the complaint that covered_nipple and erect_nipple suggest the same thing, clearly, they do not. An erect_nipple would imply either cold or arousal in the subject so that the nipple is standing out from the flesh o the breast around it, while a covered nipple is a nipple that has something hiding it from direct view.
Just because a few covered_nipple images also happen to require erect_nipple does not make them identical in any way.
In this case, there is no implied tag that both give, but rather that an erect_nipple always implies 'nipple' due to visibility, while covered_nipple does not: it is possible for a covered_nipple to also require a 'nipple' tag as well, but not in all situations.