From my experience fuel from the tank backs-up into the muffler when the engine goes from high RPM to idle. This fuel is not burned in the engine but just goes out the muffler.
BTW, if you're seeing raw fuel coming out the muffler on idle, it really shouldn't be coming from the pressure line. For the fuel to back out of the tank would require a couple of things. First off, it's really a long way from the pressure outlet in the tank to the muffler. And it'd take some pressure to push that fuel that far. And it'd have to do it against the engine's pressure that's reduced on idle but still measurable. And that pressure would have to come from somewhere, and the only external force would be from the line to the needle valve and that's sucking when the engine is running. It's possible that at full throttle the muffler pressure blows up a plastic tank and when the muffler pressure drops at idle the tank shrinks back and provides pressure, but that's really improbable. And you can easily check that out on the bench.
Raw fuel is part of the exhaust of almost all model engines. They're designed that way so the unburned fuel carries off a high proportion of the heat that's blown out the exhaust. There's a good chance you don't see it at high throttle since the exhaust is at high velocity. And some of the heavier solids might collect inside the muffler during your middle throttle running in the pits. Or during high speed running in the pits. And when you then chop your throttle the different exhaust velocity actually works better to push out those droplets.
It's not odd to see drops of fuel blowing out the exhaust at idle for some engines.