[labelling] Fix masked symbol layers loss when

- opening layer properties window; or
- changing label settings in style dock

python/gui/auto_generated/qgstextformatwidget.sip.in

@@ -159,6 +159,8 @@ Controls whether data defined alignment buttons are enabled.
 
 
 
+
+
   protected slots:
 
     void updateLinePlacementOptions();

src/gui/qgstextformatwidget.cpp

@@ -843,6 +843,7 @@ void QgsTextFormatWidget::updateWidgetForFormat( const QgsTextFormat &format )
   mBufferEffectWidget->setPaintEffect( mBufferEffect.get() );
 
   // mask
+  mMaskedSymbolLayers = mask.maskedSymbolLayers();
   mEnableMaskChkBx->setChecked( mask.enabled() );
   mMaskBufferSizeSpinBox->setValue( mask.size() );
   mMaskBufferUnitWidget->setUnit( mask.sizeUnit() );
@@ -1021,6 +1022,7 @@ QgsTextFormat QgsTextFormatWidget::format( bool includeDataDefinedProperties ) c
     mask.setPaintEffect( mMaskEffect->clone() );
   else
     mask.setPaintEffect( nullptr );
+  mask.setMaskedSymbolLayers( mMaskedSymbolLayers );
   format.setMask( mask );
 
   // shape background

src/gui/qgstextformatwidget.h

@@ -189,6 +189,9 @@ class GUI_EXPORT QgsTextFormatWidget : public QWidget, public QgsExpressionConte
 
     //! Associated vector layer
     QgsVectorLayer *mLayer = nullptr;
+
+    QgsSymbolLayerReferenceList mMaskedSymbolLayers;
+
   protected slots:
 
     //! Updates line placement options to reflect current state of widget