qproperty-alignment in styleSheet of QLabel with text only doesn't affect actual "alignment" property of QLabel in properties menu in the right part of Qt Creator. Also, it's impossible to manually change alignment by itself in that menu, it's only possible to change the horizontal alignment or vertical alignment apartly choosing items in the lists, but it's impossible to write down a needed alignments in the line "alignment" by yourself.
This issue occures in QPushButton too, so it is a general problem.

I have used these styleSheets for my QLabel:

QLabel

{ qproperty-alignment: 'AlignRight | AlignVCenter'; }

QLabel

{ Qt::Alignment: 'AlignLeft'; }

QLabel

{ Qt::HorizontalAlignment: 'AlignLeft'; }

QLabel{
qproperty-alignment:

{int(QtCore.Qt.AlignRight)}

;}

None of them affects this "alignment" property.