I hadn't had time to actually throw it at KOReader yet, but I had taken a look at it in Sigil, and besides Sigil throwing a fit at it when opening it, the OPF seemed correct

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Are you, by chance, using Calibre to send the book to the device? I've seen the metadata management do weird and wonky thing to the OPF sometimes, so, if possible, compare your scrambled copy's OPF to the one actually on your device...