If you are pouring fuel into the carb to prime it like that, you are doing it wrong, and no its not unusual for it to percolate fuel like that.

Doesn't matter what material the manifold is made from, the only reason they use plastic is for hot soak after running and letting engine sit and then attempting a restart. Once running the evaporation of the fuel will cool the intake to the point of collecting condensation depending on temp or even frost.