I have the 'Show Filter Bar' button on the toolbar. If I click it when the filter bar is closed, it opens the filter bar. If the filter bar is already open when I click the button, I'd expect it to close the filter bar, but it doesn't.
See https://phabricator.kde.org/D23232, which did this for the Search action.
Already fixed.
Interesting. The button didn't toggle the filter bar, only opened it. I removed and re-added the toolbar button and now it toggles.