I notice that when I add a new item to a collection, from an existing sentence, the new item inherits the current percentage of the original cloze, whatever that might be. I think it is reasonable to have it start out at 0 %.
I thought I had found a workaround: before creating a new item, I temporarily reset the progress of the current sentence to 0% before creating the item, and afterwards I set the progress percentage back to what it was before.
However, while this clumsy process appears to work, it does not change the next review date, which means that the new item may still not be up for review until much later. Frustration after frustration!
A follow-up: I see it is now possible to edit the completion percentage while creating a new item from an existing cloze, which makes the workflow a lot smoother. I still don’t see why the percentage is inherited from the original, though.