Yes, Send-to-Email makes use of metadata plugboards.
However, updating metadata in the AZW3 component of a dual-MOBI (contains both legacy MOBI and new AZW3 components) has never been implemented.
So if some of your files are dual-MOBI, then that would explain your problem.
(This is due to the difficulty in robustly modifying a potentially limited-size dual header.
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