Is the fact that the Close button on a PO does not appear when in a Pending Approval status a piece of native functionality?

I think it's possible to include that button if you edit the workflow.

Do you mean to create a workflow? I tried this out in a dev account and there were no workflows or scripts created.

I'm not sure why there are no workflows showing up there. Maybe you can't see/edit the standard workflows I'm not sure. But I've been studying Suiteflow and basically you can create a new workflow wherein you can add a button to each state like Approve, Reject etc. of the workflow(in this case the Pending Approval state). I suggest reading the Suiteflow NetSuite doc and try it out yourself there are example workflows at the end of that doc that you can follow step by step.

This is a dev account - I just turned on SuiteFlow in this environment. I have never heard of standard workflows. I was able to create a workflow that would update the line item's closed checkbox, so that solved the issue.

By standard workflows I mean the workflows already built in your account. Kind of like of how you have standard forms that you can only copy to create your own forms but you can't edit the standard forms. I'm thinking the standard workflows already built in are intentionally hidden to prevent messing with the basic functionalities but I'm not sure. Glad it all worked out.

Interesting insight. Had not really thought of it that way. Would make sense that they would leverage their own infrastructure.

I've come across similar limitations like how you can't edit the permissions of an admin role probably so people won't mess up the basic functionalities from testing on their own so I'm thinking they apply those precautions on some other areas. I suggest checking the SuiteFlow doc and trying out those samples I mentioned. It's abit technical but thankfully it's step by step w/ screenshots. The NS docs are thankfully straightforward in most cases which I really appreciate