When using "expanded" => true on a choice field that contains groups, the default form theme simply renders all radio buttons/checkboxes sequentially, without headings or any other indication of the group structure (at least under Twig).

From what I can tell, this issue stems from the fact that the choices in an expanded choice field are written directly from the FormView's choices variable, which is structured into groups. By contrast, expanded choice fields are represented using a series of checkbox/radio button fields, which are treated as children of the choice field - the problem being that the children are not arranged in any hierarchical structure.

The FormView for an expanded choice field does still have a choices variable like that of a collapsed choice field, but currently there is no effective way to match up the ChoiceView object with the corresponding child FormView in the template, so rendering the fields from the choices variable as with a collapsed choice field is not a viable option.