The CmsUiContext->theIFillInTheDropdownWith() method was written
primarily for TreeDropdownField, which don't have a select tag (only an input tag).
The method currently fails for CMS dropdowns (Dropdown form field class),
since they have a hidden select tag.
I've checked through core feature files and confirmed that every use
of the method relates to TreeDropdownField, which is why this bug hasn't ocurred earlier.