There's no metadata.db conflict; I assumed that must have been the case, but it turns out no. For what it's worth, the content server _only_ reads the DB, it never (at least, not under my control) writes to the DB.
I have resolved the issue, however; it turns out I'd misunderstood how and when calibre writes to the DB; it writes on exit, not on edit, which meant that my changes had not persisted until I quit Calibre on my editing host.
*facepalm*
Thanks for the feedback.
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