Text style when QPlainTextEdit is inside a QGroupBox

When a QPlainTextEdit is in a checkable QGroupBox, QPlainTextEdit's text style isn't well actualised when QGroupBox is being (un)checked.

To be more precise, if the QGroupBox is checked before compiling, the text will stay in black when the QGroupBox is unchecked while it's supposed to be in gray (this action is supposed to disable every widget inside the QGroupBox). In the other way, QGroupBox's text will stay in gray if QGroupBox is unchecked when compiling.

I show in screenshots every step : the first on the left is what I have in QtCreator, the second on the middle is what I get when launching my app and finally the last one is what I get when I click on the QGroupBox's checkbox -> QPlainTextEdit's background has the right color while the text doesn't.