I've often wondered how people decided on the pronunciation of characters.
Apparently, huǒ + shī + cùn + rén or yí can equal yù
Quite how this decision was reached is beyond me!
尉: Main pronunciation 主要发音: wèi Other pronunciations 其它发音: wèi,yù,yùn
示shì (actually a changed form of 火huǒ, I read)
We know that, in ...
The two syllables are related.「尉」(Baxter-Sagart OC: /*ʔut-s/) has two Mandarin descendants, which are approximately yu and wei.
Several OC syllables with initial /*ʔu-/ basically evolved into Mandarin w and Mandarin yu; you can see the same phenomenon with the phonetic component「𥁕」:
This is probably not the answer you're looking for, but both「秘」and「泌」underwent the same kinds of shifts (b > m) in certain topolects, so I would just treat it as a simple phonological shift within the labial consonant group (rather than something more "exciting"). I do not believe that the comments on it being a "Mandarin thing" are correct.