Ominous Item Spawners store their item nbt under "item" likely causing /item and /loot replace to be unable to affect them in any slot, the "contents" slot included, as other entities store their item nbt under "Item" capitalized (item, item_frame, etc)

Steps to Reproduce:
1. /summon ominous_item_spawner ~ ~ ~ {spawn_item_after_ticks:50,item:{id:"stone",count:1}}
2a. /item replace entity @n[type=ominous_item_spawner] contents with dirt
2b. /loot replace entity @n[type=ominous_item_spawner] contents loot blocks/dirt
2c. /execute if items entity @n[type=ominous_item_spawner] contents stone

Observed Results:
The ominous_item_spawner and the item it drops remain stone; the commands in step 2 fail
2a. "No targets accepted item [item] into slot 0"
2b. "Dropped 0 items"
2c. "Test failed"

Expected Results:
2a-b. The ominous_item_spawner and the item it drops change to dirt (or whatever is set)
2c. "Test passed"

While "contents" is the expected slot for this entity's nbt, the same result occurs with every other slot specifiable
This is overall behavior likely due to the fact that ominous item spawners store their item under "item" instead of "Item" with a capital 'I' like other entities do