The affricates written ch and j in English are not part of the inherited Semitic consonant set, and no letter of the twenty-two writes them. They came into Mandaic with loan words, from Iranian first and later from Arabic and Persian, and the script had to accommodate them.
Ash with the affrication mark is the answer. One letter, marked, covers what would otherwise need new signs, and the alphabet stays at its inherited length. The same device supplies f on ap, the fricatives on ag and ak, and the two th sounds on ad and at.
There is a consistent principle here across the whole family of scripts in this atlas: mark rather than add. Syriac did it for Garshuni and for the Sogdian and Persian borrowings; Turoyo does it with three dots; Mandaic does it with a stroke beneath the letter. Adding letters would unsettle the order, and in the alphabets that use their letters as figures it would unsettle the arithmetic too.
Mandaic did add signs twice, both times from ligatures of letters it already had, and once more by borrowing a letter from Arabic outright. So the rule is not absolute. But the twenty-two inherited letters were never touched, and every addition sits after them, which keeps the block that the whole family shares intact and countable.
The marks remain optional, and ordinary writing often leaves them off.