No answers so far, but maybe you didnt write/save the tags to the database?
I use tags a lot in Digikam, but once you start a hierarchy of tags, the problems start.
Renaming tags, and moving tags around the hierarchy is tricky, as you need to be sure you have updated every image with the new tag location. If you don’t then the next time you start digikam, it will read the images and recreate the tags in the old location.
I played around with this, and ended up with “orphaned” tags. These are in the database but not on any image, so you cannot delete them. I have tried deleting the database and restarting, but the orphan tags keep coming back - you see the list of them scrolling past during startup.
I raised this on the digikam.org site a few years ago, but didnt really get a response.