The issue is the inconsistent styling of dropdown menus within Moodle.
 
An example of this can be seen on the Moodle calendar page where two adjacent dropdowns exhibit different visual styles. One dropdown is a native `<select>` element (the course filter), while the other is a Bootstrap dropdown wrapped in a `<button>`.
 
https://school.moodledemo.net/calendar/view.php?view=month
 
Although they appear visually similar when closed, their underlying HTML structure differs, leading to inconsistencies in their presentation when open and potentially the behaviour.