Turn with flat above under slur

[Knute Snortum]

Hi all,

I am trying to engrave a turn with a flat above it, all under a slur.
I can't seem to get it no matter what I try.  Here is a fairly minimal
example:

%%%
\version "2.22.1"
\language "english"

\relative c'' {
  \key ef \major
  \time 12/8
  \partial 8 bf8( |
  g'4
  \once \override TextScript.avoid-slur = #'inside
  c,8^\turn^\markup { \flat } c'4 g8 bf4. af4 g8) |
}
%%%

The turn is inside (below) the slur, but not that flat. It seems like
the slur is leaving enough room for the flat, but the flat won't go
below the slur. Any suggestions?

--
Knute Snortum