Argh, the far too frequent issue that I make long confusing sentences struck again (I am not native English speaker, but that’s a poor excuse). What I did was the following: First I created a new second level tag and one additional child tag. I did not assign these tags to any images. Then I moved the new second level tag together with it’s child to another top level tag. This move produced the command line output I posted earlier. Then I restarted digikam and the two tags I moved remained at the new position.
So I repeated the same procedure but now I first assigned the tags to be moved to some random images. Again the move persisted. However I can also confirm that it does not change the tags in the image metadata - which clearly is a bug. This can easily be resolved by either writing the image metadata to file or using the maintenance tool - nevertheless it is definitely a bug in itself.
And I can say that I am using internal mysql for a long time now and didn’t have problems (which doesn’t have to mean anything, that I use just a subset of the functionality present).
EDIT:
I couldn’t find any existing bug about metadata adjusting yet, I will open it.
The mysql error I posted before is: 383326 – Moving a tag does not update the parent tag correctly
EDIT2:
File the issue about moved tag not being applied to file metadata: 386224 – Metadata is not updated when moving tags