For the past two releases all snaps have been using the old Adwaita theme since that’s what’s shipped with gtk-common-themes snap as I understand it and creates not matching themes between snaps debs and flatpak packaged apps were snaps use the old Adwaita and debs and flatpaks use the new one.
Please update the Adwaita theme in that snap that contains themes, it is so annoying to look at that old theme.
Edit:
I’ve opened up the issue at the repo:
https://gitlab.gnome.org/Community/Ubuntu/gtk-common-themes/issues/17