I haven’t had any other reports of this happening. On my test site with a impression limit of 10 the advert stops showing correctly. So the plugin code is probably fine.
Logically it’s because of some kind of caching setup. Everything gets stuck then.
Even though you say you don’t use cache. Some hosting companies use a server side thing like Varnish which often can not really be turned off. Or you may simply be looking at a browser cache – If your config for that kind of stuff is just a bit inefficient or weird that may happen. But that’s rare really.