Gtk4-update-icon-cache: error building GTK in Flatpak manifest

Hello, I have a flatpak manifest that needs to build GTK with custom flags (atoft/Reactions - dev.atoft.Reactions.json at main - Reactions -

It works fine until I bump the runtime version to 41 or master, then GTK fails to build with the following:

Running custom install script '/run/build/gtk/_flatpak_build/tools/gtk4-update-icon-cache -q -t -f /app/share/icons/hicolor'
--- stdout ---

--- stderr ---
gtk4-update-icon-cache: Failed to open file /app/share/icons/hicolor/.icon-theme.cache : No such file or directory

FAILED: install script '/run/build/gtk/_flatpak_build/tools/gtk4-update-icon-cache -q -t -f /app/share/icons/hicolor' exit code 1, stopped
FAILED: meson-install 

Any ideas what may have changed in GTK or the Platform that could stop this working? Thanks