When using the following command in a command block that has an empty chest on top, it correctly adds a stack of iron ingots.

/data modify block ~ ~1 ~ Items append value {id:"iron_ingot",Count:64b}

ACTUAL
Executing it another time does nothing, but the output still says that the block was modified.

EXPECTED
Given that it works the first time, I would expect it to add another stack of iron ingots to the chest in the first empty slot.