Look, there is a question mark next to this checkbox - hover over it and a description of how this checkbox works will appear.
In short, then:
in the order 2 items of product XXX, and 3 items of YYY
after sending the request, 1 product has been updated XXX
if this checkbox is checked, then 1 more position XXX will be found in the process and deleted, because position XXX has already been updated in this query. And YYY products will not be deleted. they were not updated in this query
And now you get the same situation - those products that did not come to update after such a request were not deleted
By the way, you can refresh your memory and see the task for which this setting was added
https://box.webproduction.ua/admin/customorder/issue/2015002152/edit/