Sometimes the prepared-toggle-feature-icon hangs.
When you work with your spellbook and press the star to toggle the spells state, the information is sent to the server and the spell do in facet change state, but the icon for the star won't be updated and you cant use the filter-button to view only the prepared spells. As for now, you have to toggle the spells and if the icon won't change, do a page reload and the correct state should be loaded. You can change all the spells and then do a reload.
Spamming the icon/button wont help.
Im sorry for this and Im trying to figure out why it happens. Google Chrome seems to work best and as always, IE / Microsoft Edge is going it's own way.