Hmm, from memory I'm almost sure we had it working in past, there was the menu item's title shown if it was not empty, or a line was shown if the title was a space only. But I might remember wrong or mix up J3 and J4.

the class in the dropdown is a divider

metismenu.mod-menu .mm-collapse>li.divider:not(.parent) {
    width: auto;
    height: 1px;
    border-bottom: 1px solid #ced4da;
}

In J3, the separator is text as it is in your example and is unchanged for non-metismenus.

In Metismenu it was intended to have a separator line (texts can be added to a menu as headlines).

We can change this so that the separator is a line if the param menu_text is "no" and is a text if the param menu_text is "yes".
But it is not self explaining.