Mandaic has been a minority language for its whole documented history, spoken by a small community inside much larger linguistic worlds, and its vocabulary shows it. Iranian words entered the language early and in quantity. Arabic and Persian have shaped the modern spoken dialect heavily.
An alphabet fitted to one language does not fit another, and the sounds arrive before the letters do. Mandaic answered with the affrication mark rather than with new letters, so marked ap writes f, marked ag a voiced velar fricative, marked ak the sound in loch, marked ash the affricates of ch and j, marked ad and marked at the two th sounds. Six or more of the inherited letters do double duty this way.
The choice not to add letters is worth noticing, because Mandaic did add signs at other times: two ligature letters and, later, a borrowed Arabic one. So the alphabet was not treated as untouchable. What the marks bought was that the inherited twenty-two stayed intact and in order, which is the part of the alphabet the tradition counts and recites.
There is a limit to how much this can be pushed. The marks are optional and are commonly left off, so a page of Mandaic often does not distinguish a loan word's sound from the inherited one it is written with. The reader supplies it, which means the reader has to know the word already.