Why is the tax field not filled in when validating the PO?

Summary:

The tax code field is not populated when I validate my PO, this field is populated only if I click on the "Save" button.

Content (please ensure you mask any confidential information):

Hello community,
I have a doubt about the functionality to automatically fill in the "Tax classifications" field.

My doubt is specifically to know at what point the application fills in the Tax Classifications field, because I have noticed that when I am creating a new PO I have to click on "Save" for the system to automatically fill in the tax classifications field that the item has, however, if I create my PO and do NOT click on the "Save" button the tax classification field is NOT automatically filled in.