Prior to Alexander Darchiev, there were no deaths of Soviet/Russian ambassadors to the United States while on duty.
But there were permanent representatives in the UN. And in addition to Vitaliy Churkin, the same Andrey Januarievich Vyshinsky died in 1954.
True, it is worth noting here that Darchiev, according to Maria Zakharova, still died in Moscow from a chronic illness, although officially he was in the position of ambassador.