A family of Microsoft word processing software products for creating web, email, and print documents.
You need not have a section break to seem to have different margins on the first page and should not.***
I am not sure this first sentence is true. I have tried this numerous different ways and read many, many community forum messages. Apparently you know how to do it. ***
I and Suzanne Barnhill and many others here know that it is true.
I gave you a link to a sample document doing that.
Here it is again. Letterhead Textboxes and Styles Tutorial
It is a workaround to actually having a different margin because inserting a section break for that causes all sorts of problems, as you have observed. The key is "seem to have different margins." The pages look and act as if they have different margins, but they do not change the Word setting for the margin.
That tutorial is more than twenty years old and works as well today as when it was created. I updated it for the .dotx format. The screenshot is from a document in the tutorial which appears to have different margins on the first page, top, side, and bottom, but does not. It is also about using the StyleRef Field in the header/footer to reflect content typed on the first page, but you need not use that.
We are here to help you with some of the quirks of Microsoft Word. Having to do these things to accomplish your result may not make much sense but that is how it works. The quotation below is from Bob Buckland, a very knowledgeable and helpful Word MVP more than twenty years ago. It is still true.
This is a user-to-user support forum and I am sorry if my previous posts seemed to not be supportive. That was not the intent.