Other than it being Massoretic, is there a reason as to why, in most cases, (See Avot Derabbi Natan 42:5 for list of exceptions) the Torah chooses to write the word for the fem. 3rd person as הוא using a vav instead of writing it as היא with a yod?
I ran a word search using sparks with the word הוא. It seems that with a possible exception of Iyov 31:11, which has a kri / ketiv, there are no cases outside the Torah having the spelling of הוא for the fem. version.
(Note, that I ran the search using only the spelling הוא on its own without any leading conjunctions, etc., as this is the only item of interest in this question.)