I asked at https://discourse.gnome.org/t/gtkpicture-gtklabel-in-horizontal-gtkbox-hexpand-and-halign-ignored/20967 and then opened a bug report at https://gitlab.gnome.org/GNOME/gtk/-/issues/6705.
The solution I could find was to replace GtkBox by GtkGrid.
I asked at https:/ /discourse. gnome.org/ t/gtkpicture- gtklabel- in-horizontal- gtkbox- hexpand- and-halign- ignored/ 20967 and then opened a bug report at https:/ /gitlab. gnome.org/ GNOME/gtk/ -/issues/ 6705.
The solution I could find was to replace GtkBox by GtkGrid.