I believe this is not really a GTK bug (even though it probably was triggered by the GTK 3.8 update) because it doesn't affect the Adwaita theme.
New GTK versions do break GTK themes but I'm not sure anything can be done about that. Hopefully, they'll eventually get GTK where they want it and maybe the GTK 4 series won't be so disruptive.
I believe this is not really a GTK bug (even though it probably was triggered by the GTK 3.8 update) because it doesn't affect the Adwaita theme.
New GTK versions do break GTK themes but I'm not sure anything can be done about that. Hopefully, they'll eventually get GTK where they want it and maybe the GTK 4 series won't be so disruptive.