The minecraft:ominous_item_spawner entity introduced in 24w13a doesn't load the spawn_item_after_ticks field when being read from NBT. This means that they instantly drop their contents when reloaded, and they cannot be summoned.

The code in OminousItemSpawner#readAdditionalSaveData appears to have a typo when reading the field (it calls the method, but does not store it back into the field on the entity):

protected void readAdditionalSaveData(CompoundTag lvt1) {
        ItemStack lvt2 = lvt1.contains("item", 10) ? ItemStack.parse(this.registryAccess(), lvt1.getCompound("item")).orElse(ItemStack.EMPTY) : ItemStack.EMPTY;
        this.setItem(lvt2);
        lvt1.getLong("spawn_item_after_ticks");
}