There are options in the "Fetch Metadata" function to require photos to be part of a particular collection, or to exclude them if they are not part of a particular collection. If you set these filters to be one of the supplied Smart Collections, they may not work as you expect.

Smart Collections are usually updated in realtime so, as the results are being applied, the content of the smart collection may change. It can also happen that sometimes the contents of the Smart Collections lag behind these changes. In short, using a smart collection as a filter in the Fetch Metadata dialog may misbehave if the smart collection depends on the Alamy metadata being downloaded.

A simple workaround for this is to create a temporary, static collection based on the contents of the Smart Collection. The contents of this will not change during the download, and you can remove the collection afterwards.