while holding a pickaxe in the mainhand, the loot from the furnace will not appear in Slot 2. The same happens when using container.1 as the slot, but using container.0 as the slot works as expected.
I did some more testing, and it seems that Duration + WaitTime has to be 6 or higher for the area_effect_cloud to apply its Effects, unless Duration is 0.
I have the same issue on 1.15.1 with furnaces. When using the command
while holding a pickaxe in the mainhand, the loot from the furnace will not appear in Slot 2. The same happens when using container.1 as the slot, but using container.0 as the slot works as expected.
I find this rather weird, because using
does insert a furnace inside Slot 2.