As you might be aware, when you loop through a page list, the steps inside the loop automatically inherit the page from the list as their step page.  That is the page that will be passed to pxForceCaseClose & which it will reference as it's Primary page in its step 4, when it is checking if the current work object is what is being closed.  If your pagelist is a set of pzInsKey then step 4 will test true, even though it is actually not so.

If the operation works when you are not in a page list, then you might try explicitly setting the step page of the pxForceCaseClose step to Primary or Top in your loop, so that you run that step in a similar context as if you were not in the loop.

The following is a good review of page names:

https://community.pega.com/sites/default/files/help_v55/concepts/concepts2/pagenames.htm

Pass the commit parameter as true. Or you can handle the same outside the loop as well. 

I think it should be passed as a parameter thus treating each workobject as one transaction.

-Saikat