3 possibilities
Just to be sure, are you really sure its the sniffer valve, and not the overflow/drain for the mechanical fuel pump? On the surface, that might seem like an annoying question, but they probably both have drain lines leading down to some place to dump any fuel that comes out, so it may be easy to confuse them.
So, if by chance you mistook the mechanical fuel pump drain for the sniffer valve drain, then the symptom you describe probably indicates a failed mechanical fuel pump.
If, on the other hand, it really is the sniffer valve, then what that means is that fuel is being pumped into the engine through the injectors, and is draining into the intake plenum (in the sump), and then out the sniffer valve drain. So, the next question would be: will it do this if the fuel injection servo is in "idle cut-off"? If so, I think you have a bad fuel servo that needs service. It is allowing fuel to pass through it when in idle cut-off. If you only get fuel out the sniffer valve with mixture rich, then that means you are priming the engine with the boost pump, to the point of flooding it and it drains out into the sump and then out the sniffer valve. While I would not call that "normal", I think a properly functioning system will do that if you leave the boost pump on long enough with the mixture rich.
Now that I think of it, there may be a third possibility. It may be possible for some leak to form in the fuel injection servo that would allow fuel to go directly into the sump WITHOUT going through the injectors. I'm not familiar enough with the servo to know if that is possible or not.
Others feel free to correct me if what I said here is wrong.