Any chance somehow a default AF value crept in there before you mounted it. I keep reading about rather strange interactions between default values and AF values if the default is set BEFORE putting in the lense AF value. Not sure I believe them, but some claim they end up additive them.
If the default is zero, ignore this suggestion, but if not consider deleting the lens's AF entry entirely (not just set to zero), change the default to zero, and try again.
One day I have to experiment to see if those postings (mostly on dpreview) about their interaction are valid.