Index: GraphicsMagick-1.3.45/coders/jxl.c
===================================================================
--- GraphicsMagick-1.3.45.orig/coders/jxl.c
+++ GraphicsMagick-1.3.45/coders/jxl.c
@@ -657,7 +657,7 @@ static Image *ReadJXLImage(const ImageIn
                       ThrowJXLReaderException(ResourceLimitError,MemoryAllocationFailed,image);
                   }
                 grayscale=MagickTrue;
-                pixel_format.num_channels=1;
+                pixel_format.num_channels=image->matte ? 2 : 1;
                 pixel_format.data_type=(basic_info.bits_per_sample <= 8 ? JXL_TYPE_UINT8 :
                                         (basic_info.bits_per_sample <= 16 ? JXL_TYPE_UINT16 :
                                          JXL_TYPE_FLOAT));