If you sell multiple of the same material, same quality sculptures to a land trader, and they end up on the same trade animal, they will lose their art tags.
So if you buy them back (or the trade animal dies somehow), you will end up with sculptures with no Art. Wouldn't be surprised if this happens with other things as well.
Bug confirmed. I have listed it on Mantis here: https://ludeon.com/mantis/view.php?id=2679 Thank-you for reporting this.