SUMMARY:
when a photo/video files has a matching XMP file, and the current setting is not to use XMP files, Digikam can't remove old tags because it rereads old XMP on every save/sync.

STEPS:
- a test media file with a matching XMP file.
- Digikam settings: use XMP files.
- Change Digikam settings: Metadata - Write to sidecar files - Write to XMP for read-only item only.
- Create tag: TEST_TAG_OLD
- "write metadata to file", sync
- optionally: open file exporer, confirm that XMP is created and "date modified" is later then the media file.
- optionally: grep - XMP file mentions TEST_TAG_OLD

- Change Digikam settings: Metadata - Write to sidecar files - Write to XMP for read-only item only.
- For the same media file, add tag: TEST_TAG_NEW and remove tag: TEST_TAG_OLD
- save/sync
- confirm that media file has "date modified" newer then the XMP file.
- optionally: check with exiftool that the media has TEST_TAG_NEW tag.
- optionally: grep - XMP file only mentions TEST_TAG_OLD

OBSERVED BEHAVIOR:
- After the sync, Digikam rereads XMP and restores the TEST_TAG_OLD tag for the media file.

DESIRED BEHAVIOR:
- Media file needs to have tag TEST_TAG_NEW and no TEST_TAG_OLD

JUSTIFICATION
- If a user changed a tag, the new tag should be in affect, regardless of prior configuration setup.

WORKAROUND:
- remove XMP file or rename its extension.