The Details form element ships with the collapsible option straight from the core. This includes the collapse library and css. Unfortunately for some reason the basic template is not working. The structure is not correct in order for collapse library to properly de/collapse the element. In order for this to work one must use Classy as base theme or copy its details.html.twig template.
This is IMHO a bug since the collapsibility of details is a core feature and therefore should be working without any additional templating.
Proposed solution - use the template that is currently in Classy theme and move it into the system module. This will render the Classy template obsolete, hence it should be removed.