I was recently in the mine at shor's stone after clearing it and it seems to contain 3 ebony ore veins which surprised me as it is marked as an iron mine on every source i have seen on it and this occurred during several playthroughs.

Is this a lucky bug or is it supposed to have those ebony as well?

The smith says that it was always an Iron mine but they started finding other ores just before the spiders moved in

Its always ebony.