@
ericmjones - I'll add 'detach/re-attach library' to my list of "try this" solutions. I normally have an inkling of why "this" is worth "trying" - but on this occasion I haven't a clue.
Some sort of ephemeral 'hook up' between calibre, your library, and windows or something; that was 'torn down' by detaching the library from calibre - weird
I wonder if something to do with the 'lazy writing' of book folder .opf files, or Find Duplicates Marked Exemptions went haywire - I can '
vaguely imagine' that a detach/attach library sequence might overcome such a glitch.
BR