The image-related sub-fields had disappeared from where you create them… eg.
WP Admin > Custom Fields > “Front Page Boxes” > Flexible Content Field.
They had also disappeared from Pages > Edit > Front Page > “Front Page Boxes” FCF.
Yet, they still appeared on the front-end and the database (as above), until I pressed update.
Anyway, I gave up trying to explain what was wrong and have just spent two hours creating it all again.
Thanks for trying to help. There was obviously an error that corrupted it all, but not knowledgeable enough on the mechanics to put my finger on it.
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