Created attachment 165780 [details]
digikam log file

SUMMARY
Digikam fails to write any XMP metadatas to file if the file contains a "[XMP-GCamera]   Hdr Plus Makernote" tag (from Pixel5 camera).

NB. May be that explain some non-reproducible behavior I had into some of my last reports since this tag is only present if the picture has been taken in HDR mode.

Exiftool -xmp:all -G1 of such picture displays:

[XMP-x]         XMP Toolkit                     : Image::ExifTool 12.56
[XMP-Container] Directory Item Mime             : image/jpeg
[XMP-Container] Directory Item Semantic         : Primary
[XMP-Container] Directory Item Length           : 47601
[XMP-hdrgm]     Version                         : 1.0
[XMP-GCamera]   Hdr Plus Makernote              : SERSUAPvZDVtXnAeLOrjTFc6b6B6wjSJaN4CN/jukAi/FEB43YR1RuBjbBjnsSoQbU9IaVSYwWdUFjSNf/4pb6KcLnQT5uWUMKBJslq/CztBrXLulGmElv2OHfeC0G1y1P8pXjtvM6KLozBiyKHX/
<***** and a very long string *****>

See attached the digikam log file displaying what append when I try to add a Title to the picture (via the Caption side panel). In short, we have this log message :

digikam.metaengine: Exiv2 ( 3 ) :  XMP Toolkit error 102: Unknown namespace prefix for qualified name
digikam.metaengine: Exiv2 ( 3 ) :  Failed to encode XMP metadata.

I don't know if that can/should be fixed but anyway there is no error/warning shown to the user.

NB. Exiftool shows this metadata but I have not been successful to remove it, I had to remove all XMP at once (-xmp:all=). May be I can report that to Phil.