\magnifyMusic does not affect to new voices (see the example below). If it is a normal behavior then why \magnifyMusic requires curly braces? It would be more logical to do this as \voiceOne etc. commands (without curly braces).
\version "2.20.0" \relative c' { \magnifyMusic 0.5 { c d e f \new Voice { g a b c } } \magnifyMusic 0.5 << { e, f g a } \\ { c, d e f } >> }
example.svg