PikePdf returning incorrect page count, how to get correct page count?

Question

I am using pikepdf to open a pdf file and get the page count for that file. My code is roughly:

import pikepdf
with open(file_uri, "rb") as input_pdf:
    input_reader = pikepdf.Pdf.open(input_pdf)
    page_count: int = len(input_reader.pages)

Very rarely a pdf file will give the incorrect number of pages. For example, I have a file that when opened on my laptop in a PDF viewer is 39 pages. pikepdf gives a page count of 47 pages. The metadata when I click Get Info in the file browser states 39 pages.

Unfortunately, I cannot share the file as it has protected information.

How would I make sure I get the correct page count from pikepdf? I've used other Python PDF libraries (eg pypdf) in the past but they have errors opening python files sometimes.