Created attachment 165088 [details]
Screenshots of the issue and album with images and XMP sidecars.

SUMMARY
***
Some images show additional non-existent tags in different parts of the application.
For example, if I click an image, open the Properties tab and then look at the Tags area it will show an additional non-existent tag called "_Digikam_root_tag_". All of my tags are important from side-car XMP files, but not all images appear to show this non-existent tag. The tag isn't listed in the thumbnail view nor does it show up in the list of assigned tags under Captions tab > Tags.

At the same time, in the thumbnail view, sometimes an additional non-existent tag called "Tags" appears, this again doesn't appear in the Captions tab > Tags area, but it also doesn't appear in the Properties > Keywords area this time.

In my examples, both of these issues appear on the same image but  I'm unsure if they are related or not.

I've attached the whole folder as a ZIP, if you import these as an album and have XMP side-car metadata files enabled, it will hopefully replicate the behaviour.
Also attached are some images showing the error, and the XMP side-car files on their own.

115341660_p0.png.xmp is the sidecar that causes the issue.
101800994_p0 - 無題.png.xmp appears with no issues.

***

STEPS TO REPRODUCE
1. Enable a MySQL Database.
2. Enable Settings > Metadata > Read from sidecar files
3. Download th attached ZIP, extract  it, and add it as a new album.
4. Observe the imported images and their tags.

OBSERVED RESULT
One of the imported image shows additional non-existent tags.

EXPECTED RESULT
Only real tags should appear.

SOFTWARE/OS VERSIONS
Linux/Ubuntu: 22.04.3
KDE Frameworks Version: 5.110.0
Qt Version: 5.15.11 (built against 5.15.11)

ADDITIONAL INFORMATION
Using the AppImage with a MySQL Database.