Villagers giving reverse discount

I was trading quite a few glass panes with a Cartographer, and he started to give discounts as normal, but when I kept coming back several minecraft days in a row, he started asking for way more than before. I'm not sure if it's an integer overflow or what, but he eventually tried asking for 32 panes for 1 emerald, despite the original amount being 11 for an emerald. 

And no, it wasn't a graphical error; I tried only putting fewer panes, and he wouldn't give me an emerald, despite clicking where it should have been.