Tag not written to photo file if in PEF or DNG format

I would like to have my tags written in the image file, so I checked the option "Write tags, titles, and other metadata to photo files". It seems to work with jpeg photos but not with PEF or DNG. I have restarted Shotwell.

Test case:
1. Start Shotwell having jpeg, PEF and DNG files in the library and the option above checked.
2. Add a tag to a PEF and a DNG file
3. See that the file dates do not change
4. Add a tag to a jpeg file
5. See that the file date changes

I would expect Shotwell to update the DNG file and to update the xmp-file accompanying my PEF-file.

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: shotwell 0.11.6-0ubuntu0.1 [modified: usr/share/glib-2.0/schemas/gschemas.compiled]
ProcVersionSignature: Ubuntu 3.0.0-14.23-generic 3.0.9
Uname: Linux 3.0.0-14-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 1.23-0ubuntu4
Architecture: amd64
Date: Mon Jan 2 21:27:08 2012
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111012)
SourcePackage: shotwell
UpgradeStatus: No upgrade log present (probably fresh install)