In the case of U+1D42 Modifier Letter Capital W I was wrong about the cause, it was in fact specifically added on the rationale that for this purpose (phonetics) the presentation was semantic in nature, and so the plain text (thus Unicode) needed to preserve these symbols which could otherwise be handled by a presentation layer.
U+1D42 Modifier Letter Capital W was added in Unicode 4.0 as part of the Phonetic Extensions and Wikipedia provides a long list of Unicode committee paperwork regarding this: https://en.wikipedia.org/wiki/Phonetic_Extensions
You can see that initially it would have been numbered differently and then over the course of several drafts the proposal evolved until it was assigned U+1D42
U+1D42 Modifier Letter Capital W was added in Unicode 4.0 as part of the Phonetic Extensions and Wikipedia provides a long list of Unicode committee paperwork regarding this: https://en.wikipedia.org/wiki/Phonetic_Extensions
You can see that initially it would have been numbered differently and then over the course of several drafts the proposal evolved until it was assigned U+1D42