If I open a bib-file not created with BibDesk that has entries with Doi-fields, the values doesn't show in BibDesk. And if I save the file using BibDesk, the Doi fields are stripped from the file. I've attached an example.
If I add the DOI in BibDesk, however, it is saved correctly. It seems like BibDesk only reads the Doi field if a Bdsk-Url-1 field is present too. If I add
Bdsk-Url-1 = {http://dx.doi.org/10.1103/PhysRevB.83.205101}}
to the example file, BibDesk reads the Doi field correctly.
This is not a bug. It is controlled by preference settings in the Default Fields preferences.