In "qcommonstyle.cpp', function "sizeFromContent(), under the case for CT_HeaderSection.

When adding space for the sortIndicator, it uses the pixelMetrics for "PM_HeaderMargin" instead of "PM_HeaderMarkSize".   I think that's why the bottom indicator always seems to overlap the header section's content.

 if (hdr->sortIndicator != QStyleOptionHeader::None) {
     int margin = proxy()->pixelMetric(QStyle::PM_HeaderMargin, hdr, widget);
     if (hdr->orientation == Qt::Horizontal)
         sz.rwidth() += sz.height() + margin;
     else
         sz.rheight() += sz.width() + margin;
     }
  }

I think it affects all versions.