When QHeaderView paints the section, when eliding text, it doesn't account for the extra width taken by the DecorationRole icon.

https://github.com/qt/qtbase/blob/e4c39d5e1e7ee8c2bba273e6d613ec519b7fa9c2/src/widgets/itemviews/qheaderview.cpp#L2754-L2771

The fix would be to first get the icon, if any, and add the icon's pixmap's width to the text margin, i.e.

if (!opt.icon.isNull())
    margin += get_pixmap_width(opt.icon)

if (d->textElideMode != Qt::ElideNone)
    ...

I don't know how to get actual pixmap width from QIcon as it probably depends on the dimensions of the underlying header item

Probably related issue: https://bugreports.qt.io/browse/QTBUG-30116