@rossbennett this is essentially a behavioural changed caused by switching from using the OS X metadata to search bookmarks over to indexing bookmarks internally (as in High Sierra, there is no longer metadata for bookmarks).
The new bookmarks feature in Alfred was partly borne out of necessity, and brought in later in the macOS 10.13 beta cycle than I usually like (as I had hoped that the missing bookmark metadata was just a macOS bug).
As such, this feature is still young, and has some improvements in the pipeline which should better restore the behaviour you are expecting... This will be sooner rather than later
Cheers,
Andrew