Vista - Photo Gallery bug: some thumbnails missing encrypted files

In Vista I enabled encryption on my pictures-folder. All thumbnails are
looking nice in explorer view and Microsoft Office Picture Manager. Only in
Photo Gallery the thumbnails of SOME pictures are missing. Even with
jpg-pictures taken with the same camera on the same day, some do have
thumbnails and some don't.
After disabling encryption all thumbnails look fine. Enabling encryption,
the same thumbnails are missing again.
Is this a known bug or what's happening here?
Thank you very much for your help.

Jan