The advanced scene switcher plugin displays checkboxes instead of expand/collapse icons.
This happens with all themes, including the original OBS themes (as far as I tested)
So I don't have any direct influence on it, maybe it's due to an internal change in OBS or an incoherence in the Advanced Scene Switcher layouting. IDK
Since I already had to play around with the checkboxes, I took the opportunity to make them consistent across OBS, not just in the Settings window.