I've seen ''millioneth'' a few times in older books. Dictionaries seem to all say ''millionth''. Is ''millioneth'' simply incorrect or could it be that it is an old non-standard spelling.
If you try Google nGrams then millioneth certainly looks much rarer than millionth, probably enough now to be described as incorrect
If you do a Google book search for the 17th and 18th centuries (not totally reliable), millioneth was less common than millionth, though both seen relatively rare. At that stage they might have been seen as variants, with millionth later dominating