This is getting ridiculous !
I completely un-installed digiKam, even to the point of manually deleting every last vestige from the registry.
I performed a new pristine virgin installation of the most recent stable release that I could ascertain, version 5.9.0
One by one, I meticulously reinstated my entire photo collection of finished jpegs ONLY.
Once I got the entire collection reestablished in digiKam, I closed my eyes and investigated Tags View.
THE STUPID PROBLEM IS STILL THERE.
Just for fun, I purged the metadata from a selected group that were not displaying properly.
I created some brand new header tags and brand new sub-tags.
I brought these up in Tags View and still they had the problem.
I did learn another quirk about the situation: I had a particular tag that I knew for fact should have had 22 images displaying from a certain “collection” or “Album” if you prefer.
Only five were showing; the blue header-bar said there should be eight; and, I knew for fact that there should be 22.
I removed the tag from the five that were showing and the remaining seventeen that had not been showing magically appeared with full control abilities.
Just for fun, I moved to a different HDD and that did not help a bit.
I was determined to get to the bottom of this situation, emphasis on the “was”; I think I will take up drinking instead.
So far, all thumbnails show up and work fine in Albums View.
Just about every time I use digiKam, I find more and more images wrongly tagged with various random tags that no way did I apply.
Surely I am not the only human alive that has had this issue.
The big mystery is that there was no problem at all and everything worked perfectly right up until it all went haywire.
Also, since I did the new install, digiKam now crashes frequently for no reason that I can fathom; and, when it reopens after a crash, it has forgotten where I was.
I have numerous photo editing programs and not the least problem with the performance of any of them — only digiKam.