There's a problem with dropdown menus in Wordpress 5.6 caused by out of date JQuery syntax in Vertex supplied with "Design Control" theme and probably others. The first menu drops, then no further menus work until the page is reloaded.
A temp workaround is to use the JQuery migrate helper, and enable old JQuery but Wordpress version of Vertex needs an update as this is only a temp fix.
The migrate helper log says:
jQuery.fn.size() is deprecated; use the .length property
I tried manually changing this as suggested in both the minimized and JS files, but that's not enough to fix it in my testing at least
Please would you look at a Vertex update before they kill the migrate helper in next WP release (March/April ?)