The theme needed some small adjustments for the recent expander state changes ACTIVE to CHECKED.
974807f Adwaita: Make expanders turn again
gtk/resources/theme/Adwaita/_common.scss | 4 +-
gtk/resources/theme/Adwaita/gtk-contained-dark.css | 63 ++++++-----
gtk/resources/theme/Adwaita/gtk-contained.css | 109 ++++++++++----------
3 files changed, 83 insertions(+), 93 deletions(-)
Upstream: git.gnome.org