setLayoutDirection breaks text on QPushButton

Since Qt 4.7,

qApp->setLayoutDirection(Qt::RightToLeft);

should have no effect on any rendered text. E.g. a QLabel with the text -20 should and does show that string, regardless of the layout direction. But with QPushButton, the following causes the text to be displayed as 20- :

ui->pushButton_3->setText("-20");
qApp->setLayoutDirection(Qt::RightToLeft);

The bug was reported to affect 5.4.0 by a user on the forum, see https://forum.qt.io/topic/77716. I can reproduce it with 5.8.0.