The items can’t be stacked with others if they were dropped, or if you press X or A on the other stack
From Félix Laurin: It appears that we cannot stack items from that come from different conditions. I.e: we cannot stack "Stone blocks" that come from mining with a "Silk touch" pickaxe with "Stone blocks" that come from smelting, or we cannot stack "cooked salmon" cooked by a "fisherman villager" with "cooked salmon" cooked in a furnace.