OK, @louis and @verity-davey, this is what I’ve found in my Grammar of Middle Welsh (D. Simon Evans, Dublin Institute of Advanced Studies, 1976, pp.61-62).
Basically, he says that even allowing for the vagaries of mediaeval spelling, the situation found in Middle Welsh is inconsistent. However, taking into account the evidence we’ve got, and comparing it with Old Irish, it appears to go back to a pattern where negatives in relative clauses caused a soft mutation (lenition), but in main clauses caused an aspirate/spirant mutation (p, t, c → ph, th, ch, everything else including b & m unaffected). These two separate patterns get confused, resulting in the modern “mixed mutation” that we’re used to (p, t, c get +h, everything else gets softened that can be), except that in the Middle Welsh period that merger was not complete or consistent – b and m sometimes don’t get softened.
“In the early period, the negative in a relative clause is followed by lenition of all lenitable consonants, including p, t, c… In principal clauses, on the other hand, the negative was usually followed by the spirant mutation… Only traces of an older system, however, survive in Welsh… In MW prose the earlier system has been much simplified, and the position may briefly be summarized as follows… After the negative we have the spirant mutation of p, t, c, and the lenition of the other lenitable consonants, in both principal and relative clauses. Traces of the older construction survive in non-lenition of b and m after the negative : Ny byd ‘There will not be’… but lenition occasionally occurs : Ny uynhei Gaswallawn y lad ynteu ‘Caswallawn did not wish to slay him’.”
Tl;dr – It’s William Morgan harking back to Middle Welsh, in the same way that King James leans on Wycliffe.