Might be a thin layer of super glue holding the nut in place.
A light gentle tap from the side is usually enough to take them off without doing too much damage.
I would also recommend using a very sharp blade to score the edge where nut meets the fret board first to prevent bits of that coming off too, which is easy to do on rose wood type fret boards, not such a problem on maple as it is a much harder timber.
Cheers, Waz

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