Subelements of CP and Single Pages don't open on click on top element

In the CP, if a page has subpages, they don't open (drop-down) when clicking the main page. Subpages only open when clicking on the tiny triangle left to the main page. The same occurs when a CP has other CPs as subelement.
Subelements should open (drop-down) automatically when the main element is clicked

CP with CP subelements: https://multicoisas.olat.com/url/RepositoryEntry/4620527