Not sure if its a bug or intended behavior (more likely a bug... you might move it Mario)
I accidentally had MWG switched off in the metadata panel. When I change either of the mentioned fields, changes are written to the file, but the metadata panel does not reflect it.
The variables shortly shows the new values but immediately the previous values.
I need to open the files in PS and save it again to reflect the changes I did in IM.
PSD and JPG so far. On DNG it often works, but not always and is not to test by saving them again.... ;)
See the log file and the recorded session: http://samples.no-nameweb.de/Metadata_titel.wmv (9MB)
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This sounds like you updated XMP data on write-back, but on re-import the existing IPTC record was imported again, replacing the corresponding tags in XMP. This is why your values snapped back to whatever was stored in the IPTC record.