OK, in Cubase press F5 to open the Media Bay.
Open up “This Computer” …
… and navigate to the disk location of your Steinberg Content:
Right-click on “Content” and select “Rescan disk” and leave Media Bay open, as it seems to take some time.
Then go back up, open out “VST Sound” and navigate to the missing library:
Right click on Simon Philips Jazz Drums and “Rescan disk” again:
There may be quicker or better ways to do this, but that’s what worked for me.
After that, I can see 53 “items” listed.
I had to do this again after the Cubase 15/Groove Agent 6 SE upgrade, so there’s clearly still something wrong with this Expansion.