I'm not sure if it's something I've done, but I find that when I've added hierarchical keywords to images, the 'write' pencil shows pending writes for both Hierarchical Keywords and Subject. That's what I would expect. However, after the write process completes, the images are marked again as pending, with Subject as the pending tag. I would have expected both tags would have been written during the same write process.
Is this 2-step process unavoidable, or have I perhaps set some preference in a less than optimal way?
It does seen to happen when I have a combination of raw and jpg versions, not when all the images are jpg.
The usual reason for this are out-of-sync keywords between flat IPTC/XMP keywords and hierarchical XMP keywords.
Then IMatch might need two write-back/import cycles to unify the flat and hierarchical keywords. The details sometimes vary, but its usually that.
See also the corresponding section Files Pending Again After Write-Back (https://www.photools.com/help/imatch/#md_for_beginners.htm?dl=h-51) in Metadata Problems and Pitfalls (https://www.photools.com/help/imatch/#md_for_beginners.htm?dl=md_problems)