Interestingly, the OED disagrees with this (quite reasonably sounding) origin theory of lieutenant: https://www.grammarphobia.com/blog/2015/02/lieutenant.html
It says that in French of the time as heard by English people at the time, the phonetic value of that u was ambiguously heard as f or v.
My position is v and u were phonetically ambiguous or sometimes heard as the same in Latin and romance languages. Maybe also in older forms of English.