Had a bug yesterday too, regarding trade deals. Peter would initially ask for 2 luxes and 5gpt for a great work, but as I "haggled" he kept upping the price to about 2 luxes, horses, iron and 15gpt. About 3-4 turnus later I tried to trade him again, and by accident I clicked on a few iron instead of gold and then his great work. He refused the initial deal, and when I clicked "what would it take?", the trade went green and he sold me a great work of writing for 4(!) iron. He was super stocked up on iron too (60-70) so there was clearly a bug at play.
Needless to say, I cleaned him out for 4 great works before he refused selling me any others. It felt like cheating either way.
Perhaps related, but I did initially trade him some diplo favour for gold, then bought two great works for "normal" prices (when my haggling failed me) before he basically donated me everything, so perhaps there was some overflow involved or similar.