As it turns out, newest versions of GIMP may depend on babl/gegl features that are not yet released. In particular, using stable babl 0.1.108 triggers a warning «unsupported format "HSVA float"», and installing babl-git 0.1.109.r1886.2dc17be-1 fixes it.
when you build gimp from git source you need to build gegl and babl too from souce because many changes are made in the same time on all 3 repositories
As it turns out, newest versions of GIMP may depend on babl/gegl features that are not yet released. In particular, using stable babl 0.1.108 triggers a warning «unsupported format "HSVA float"», and installing babl-git 0.1.109.r1886.2dc17be-1 fixes it.
From https://gitlab.gnome.org/GNOME/gimp/-/issues/11370#note_2089398: