A task is separate to the list item. You need to be sure that the attachments control on your task form is for your list item, not the task itself.
that's right that task's attachments are independent to item's ones.
however, you can not add attachment control linked to a item's atchments on a task form.
if you need to make attachments accessible between the two you have to copy them over within workflow. see an example here collect task attachments in list item and show all attachments in next task
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