Yeah, the numbers do appear as soon as I download the file, before making/saving any edits. This is happening with different forms as well, not only I-751. It's happening with 485, 130, 765, etc..
The only way that seems to work is saving the file as a PDF directly from the USCIS website, instead of downloading it. But doing it that way doesn’t allow any edits in the fillable fields.
Please let me know if you find a different workaround!