QPushButton style changes in horiz layouts > 2 items

If you have a QPushButton with the text set to a minus sign, in a horizontal layout with 3 or more items; the styling of the QPushButton changes to a borderless version.

This behaviour occurs in Designer and at run time.

I attach images of a layout with 2 items and a layout with 3 items to show the difference. I also attach an image of a QPushButton without a minus sign to show it renders OK.

I tried to replicate this bug in Linux but did not encounter it.