Proportional strict grace notes

With strict-note-spacing spacing for grace notes (even multiple ones) is floating as well.

\version "2.24.0"

%% http://lsr.di.unimi.it/LSR/Item?id=229

\relative c'' {
  \override Score.SpacingSpanner.strict-note-spacing = ##t 
  \set Score.proportionalNotationDuration = #(ly:make-moment 1/12)
  c8[ \grace { c16 c } c8 c8 c8] 
  c2 
}