CMVideoFormatDescriptionCopyAsBigEndianImageDescriptionBlockBuffer

CMVideoFormatDescriptionCopyAsBigEndianImageDescriptionBlockBuffer

and

CMVideoFormatDescriptionCreateFromBigEndianImageDescriptionBlockBuffer

seem like some useful utilities, but... they are not acting the way I expected. In my mind I should be able to pass the first a CMVideoFormatDescription then pass the created CMBlockBuffer to the second and get out a CMVideoFormatDescription that matches the first CMVideoFormatDescription. I don't. Am I misunderstanding what these utilities are for? Here are printouts of the original CMVideoFormatDescription:

<CMVideoFormatDescription 0x600000ca7330 [0x7fff8d24f7d0]> {
  mediaType:'vide' 
  mediaSubType:'avc1' 
  mediaSpecific: {
  codecType: 'avc1' dimensions: 1280 x 720 
  } 
  extensions: {{
    CVImageBufferColorPrimaries = "ITU_R_709_2";
    CVImageBufferTransferFunction = "ITU_R_709_2";
    CVImageBufferYCbCrMatrix = "ITU_R_709_2";
    CVPixelAspectRatio =     {
        HorizontalSpacing = 1;
        VerticalSpacing = 1;
    };
    FormatName = "H.264";
    SampleDescriptionExtensionAtoms =     {
        avcC = {length = 42, bytes = 0x014d0020 ffe1001b 274d0020 898b6028 ... 40010004 28ee1f20 };
    };
}}
}

and the copy via the utilities:

<CMVideoFormatDescription 0x600000cb40c0 [0x7fff8d24f7d0]> {
  mediaType:'vide' 
  mediaSubType:'avc1' 
  mediaSpecific: {
  codecType: 'avc1' dimensions: 1280 x 720 
  } 
  extensions: {{
    CVFieldCount = 1;
    CVImageBufferChromaLocationBottomField = Left;
    CVImageBufferChromaLocationTopField = Left;
    CVImageBufferColorPrimaries = "ITU_R_709_2";
    CVImageBufferTransferFunction = "ITU_R_709_2";
    CVImageBufferYCbCrMatrix = "ITU_R_709_2";
    CVPixelAspectRatio =     {
        HorizontalSpacing = 1;
        VerticalSpacing = 1;
    };
    Depth = 24;
    FormatName = "'avc1'";
    FullRangeVideo = 0;
    RevisionLevel = 0;
    SampleDescriptionExtensionAtoms =     {
        avcC = {length = 42, bytes = 0x014d0020 ffe1001b 274d0020 898b6028 ... 40010004 28ee1f20 };
    };
    SpatialQuality = 0;
    TemporalQuality = 0;
    VerbatimISOSampleEntry = {length = 171, bytes = 0x000000ab 61766331 00000000 0000ffff ... 00000001 00000001 };
    Version = 0;
}}
}