Yes, this makes me believe it is an bind issue. Either due to a misconfiguration (I guess you already ruled that out, though) or due to a bad default value for something you don't have configured at all. This would also make sense as you say that the latter bind version doesn't show this - the default value might have been adjusted as they realized this. Maybe the ISP's DNS servers are running the same "bad" bind version resulting in this seen by the other users.