Can't define CSS with properties for QToolButton with Qt 6

The following CSS does not work with Qt 6, but works with Qt 5 on a QToolButton where popupMode is set to MenuButtonPopup.

CSS:

QToolButton[popupMode="1"] {
 padding-right: 20px;
}

According to https://doc.qt.io/qt-6/qtoolbutton.html - QToolButton::MenuButtonPopup is 1.

The UI file shows the property is QToolButton::MenuButtonPopup for "popupMode".

This is what the button looks like:

Qt 6:

Qt 5: