Some releases don't have tags while album's artists have some, and Picard is using album's tags (release/release group) to set the Genre metadata tag.
When no tag are found, Picard fails to set a Genre, i propose to add an option to fallback on artist's tags in this case.

Example: http://musicbrainz.org/release/ec102c8e-981b-4aec-b1e4-35eda11f2bb6

As now, both artists have tags, while release group and release have none, so Picard don't set any genre.
Having such feature would allow Picard to set Genre to "Doom metal".