You have your entire text formatted for "keep lines together."
That is not a good idea.
You are also using lots of blank paragraphs for spacing. Also not a good idea.
2.2 Why you should press Enter only once to end a paragraph

I don't think either of those is the source of your problem, though.

I confirmed that there is only one section break and that it is a "next page" section break rather than an "odd page" section break.

You move from page 3 (the end of your TOC) to page 5 your introduction. In the Word document this can be seen in the status bar.

I have tried removing the gutter setting. That did not change it.
I changed your ToC from 3 pages to two. No blank page then.
So, for whatever reason, Word is insisting on starting your second section on an odd-numbered page.

I did spot the following non-printing mark in your document that I cannot explain.
00 deleteme 2.png

It is right at the end of your ToC field but is not a normal part of such a field.

I would recommend that you review Numbering Front Matter (Ribbon) by Suzanne Barnhill. You may want to consult with her on this.

Rebuilding the document would be an option. If you do, do not copy the last paragraph mark of your document into the new document.

I just tried this, copying everything from the old document to the new one except the last paragraph mark. When I printed to pdf after doing that, there was no blank page. This tells me there is likely some kind of document corruption.