I have a conjecture I'm not sure if there's an easy way of testing, but I'm pretty sure when you control+shift+l (or whatever your browser uses) to change the circuit, it continues to use the session credential you got after submitting the proof-of-work, because you generally don't have to do another PoW.
I think sometimes it's bad but your browser doesn't know this or for some reason the server doesn't accept it. So instead of getting the PoW page you just get nothing.
I know it loses the session credential when you restart entirely, because you always have to do another PoW.
Usually if you just directly connect to a page on the Farms without a session credential from a PoW, it immediately fails over to the url that gets you to the PoW page. But maybe sometimes it doesn't.