I use Brave rather than Safari or Chrome, but the "Web Bookmarks" feature doesn't find my Brave Bookmarks.
macOS Sierra 10.12.6
Brave Version 0.59.35 Chromium: 72.0.3626.81 (Official Build) (64-bit)
Web Bookmarks Feature is Configured as follows:
Sources: "Google Chrome Bookmarks"
Show Bookmarks "via keyword" = "bm"
Open Bookmarks "in macOS default browser"
If I add a bookmark to brave and search for it using "bm" it is not found.
If I add the same bookmark to Chrome, it works fine.