This looks intentional, from InlineTextBoxPainter::paintCompositionUnderlines:

    // We need to have some space between underlines of subsequent clauses, because some input methods do not use different underline styles for those.
    // We make each line shorter, which has a harmless side effect of shortening the first and last clauses, too.
    start += 1;
    width -= 2;

Presuming this is intentional, so closing but at least we've a record, eh.