For the sole JPA occurrence of this JBA and LJLA form compare the note s.v.
mṭwl prep.
In LJLA such spellings instead of simple מטול can occasionally be scribal "improvements" ; however, in TgChron (always) and other LJLA texts (mostly) there seems to be a clear distinction between this form used in the sense of "on behalf of" and מטול othewise, i.e., with very few exceptions the two forms are suppletive, with this form being used with pronominal suffixes and before the names of persons or groups, while מטול rarely occurs in such a context.
Note that in Mandaic this form is found in the later manuscripts, while the early epigraphic texts use
mṭwl.