I had a bug with the Steal Treasure ability this morning too, though mine is a bit different.

I clicked on a trade link to send someone plague, offering to send the max of 5 plague while the box for the Steal Treasure ability was ticked. I then hit confirmed, the screen refreshed and I found I still had 5 plague, but 2 purity less than when I started. The Steal Treasure ability was now also on cool down.