QComboBox Doesn't Enable When Parent QFrame Is Enabled

I'm seeing unexpected behavior by QComboBox instances within QFrame parents.  At first I was trying to place the QComboBox inside a QGroupBox that is checkable.  When the parent QGroupBox is checked the QComboBox does not become enabled along with the other widgets inside the QGroupBox.  I see the same behavior when the QFrame is removed from the QGroupBox and I handle the toggled signal of the QGroupBox instance and manually set the enabled property of the QFrame to the toggled signal "on" value.

In order to "workaround" this unexpected behavior in which the child QComboBox instance is not enabled I must also set the enabled property of the QComboBox instance.

I attached a small Qt project demonstrating the issue on both Windows and Ubuntu 18.04 LTS using Qt 5.12.4.

To activate the "workaround" uncomment the following in the Qt project file:

1. uncomment to use workaround to have QComboBox enabled when parent QFrame is enabled
   #DEFINES += HAS_QCOMBOBOX_ENABLE_WORKAROUND