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Appendix probi was a book written in 3rd-4th century AD to correct people's written Latin.
It consists of lines of the form "xxx non yyy", meaning that people commonly wrote yyy, where xxx is the "correct" form.
However, a line reads:
cannelam nun canianus
Why is "nun" used here?
It was a typo.
All of the online sources I find have been based on the book Sprachlicher Kommentar zur vulgärlateinischen Appendix Probi, but in the book it was correctly printed as non (Page 5 of this document).
Therefore, I suspect that someone, when typing from that book, made a typographical error which is then spread everywhere.
On Page 73 of Lingüística Romanica, it is also correctly printed as non.