EXIF: and TIFF: metadata is missing from XMP block in Lightroom 4

Question

I've recently purchased Lightroom 4 and on the whole I'm very happy with it, but I have run into a problem.  I have a few scripts which grab information from the XML-based XMP metadata block that's embedded in the DNG and JPG files that I use.  I've found that images processed with Lightroom 4 no longer export the exif: and tiff: data.
To illustrate, a photo I exported with a previous version of Lightroom has the following data in the XMP block
   tiff:Make="Canon"
   tiff:Model="Canon EOS 40D"
   tiff:ImageWidth="850"
   tiff:ImageLength="567"
   tiff:XResolution="240/1"
   tiff:YResolution="240/1"
   tiff:ResolutionUnit="2"
   exif:ExifVersion="0221"
   exif:ExposureTime="1/800"
   exif:ShutterSpeedValue="9643856/1000000"
   exif:FNumber="71/10"
   exif:ApertureValue="5655638/1000000"
   exif:ExposureProgram="3"
   exif:DateTimeOriginal="2010-08-22T10:52:07.58+01:00"
   exif:DateTimeDigitized="2010-08-22T10:52:07.58+01:00"
   exif:ExposureBiasValue="0/1"
   exif:MaxApertureValue="4625/1000"
   exif:SubjectDistance="428/100"
   exif:MeteringMode="5"
   exif:FocalLength="80/1"
   exif:CustomRendered="0"
   exif:ExposureMode="0"
   exif:WhiteBalance="0"
   exif:SceneCaptureType="0"
   exif:FocalPlaneXResolution="3888000/876"
   exif:FocalPlaneYResolution="2592000/583"
   exif:FocalPlaneResolutionUnit="2"
   exif:PixelXDimension="850"
   exif:PixelYDimension="567"
There is, of course, a lot more information in the XMP block, but in a file exported using Lightroom 4 there is nothing in the XMP block with the exif: or tiff: namespaces so none of this information is available to any script which queries the XMP metadata.  I'm pretty sure that the metadata is still there, embedded in the original EXIF data, as I can view it using the metadata panel in the Library module, but that's not accessible to my scripts as it's not in the XMP block.  I've made doubly sure that the metadata has been saved and also double-checked all my settings and even tried to export the files multiple times with every different setting that I can think of (even though my current settings worked fine in previous versions) but the files never get the exif: or tiff: data.
I have called Adobe, but the best help they could give me so far was to post here and hope someone can help!

Answer 1

I just compared two JPEGs exported with LR 3.6 and LR 4.1RC. The tags you mentioned are exported neither by 4.1 nor by 3.6. These tags are only present in the normal Exif metadata, but not in XMP. Which version of LR 3.x did you use before? Perhaps Adobe changed something already in the later versions of LR 3.x. If this is the case, you have to use a script or tool that is able to read the Exif metadata directly.
In the XMP metadata in the file exported by LR 3.6, I see only crs (camera raw), aux (e.g. serial no.) and some other XMP namespaces, but no exif or tiff.
P.S. I viewed the metadata of the exported files with exiftool. File format was JPEG (with "quality" setting, not with "limit file size", because apparently there are still some XMP differences between these two modes).