SUMMARY
Many tags stored in photos are not inserted into Digikam's tag list when photos are read.

STEPS TO REPRODUCE
1. Install digikam, add directory where photos reside.  In this case, photos are stored on a network drive in many different folders.
2. Allow digikam to read all photos
3. After digikam completes scan of all photos, observe tag list.

OBSERVED RESULT
Tag list is incomplete

EXPECTED RESULT
Tag list is complete

SOFTWARE/OS VERSIONS
Windows: 10

ADDITIONAL INFORMATION
Used Xviv to insert xmp subject tags into many photos.  

(For example:  ./exiv2 -M "add Xmp.dc.subject 5 Star Photos: 2003 - 2005" "P:/2003/2003_01/esimg2829.jpg")

Many photos are tagged with the same subject.

When looking at individual photos, DigiKam shows that the tags exist (in the metadata view), but the photo does not appear under the "5 Star Photos: 2003 - 2005." tag in the tag view.

I think this has to do with the filename of the photo.  In many cases, the filenames of photos are similar.  For example:  "esimg374 3-19-2003 12-53-02 PM.JPG", "esimg374 3-19-2003 2-12-36 PM.JPG" 

Is it possible that there is a dependency on file name?

Also tried re-reading image metadata with the same result.