The maximum number of deaths in 1918 was in October, by far, it was a spike. I don't think many soldiers were returning home in October because the war went on another month. Even the wounded would have been treated in France, those who had been wounded seriously in July or so might have been returned to the States in some numbers.
We were bringing in some German POWs who could have had the flu. But, it was not asymptomatic for days as this COVID is.