Interesting question. At first I wanted to say yes to your question, but then I realized I better check it out before I say anything. I also somehow expected all the suffixes always to be pronounced the same way, for all prepositions, but that is the case only in some persons. For instance - "you" masculine singular - as far as I can tell will always be "cha" ending - לך lecha, אותך otcha, איתך itcha, ממך mimcha, לידך leyadcha, and so on. "you" masculine plural will also always be "chem" - לכם lachem, אתכם etchem, איתכם itcham, לידכם leyadchem and מכם mikem being the only exception I could find. But "I", for example is more diverse: לי li, אותי oti, ממני mimeni, לידי leyadi, but לפניי lefanay, אליי elay, עליי alay, but "he" is the also more diverse, as the ו can be either "o", "u" or even "v". הוא - hu, ממנו - mimenu, לו - lo, אותו - oto, איתו - ito, אליו - elav, עליו - alav.
But basically, other than the actual pronoun, ממנו is the only preposition I could find that has the "u" sound, so I'd guess it's an oddity.
(sorry for the longish post)
According to Pealim ממנו is both from us and from him, pronounced "mimenu". https://www.pealim.com/search/?q=%D7%9E%D7%9E%D7%A0%D7%95