I'm with
@Kanga on this.
I started at Le Puy in April 2016 and my first credencial showed this.
I suffered an injury and retired hurt at Estella / Lizarra.
As my first credencial was almost full, and had some sello as I tried to continue after Estella, i decided to get a second credencial and returned to Estella in October 2017.
The hospitalera there filled in the first page showing Estella as the start point.
At Santiago it was all irrelevant.
For the Compostellana the focus was only on:
"Did I walk all the way from Sarria?"
For the certificate of distance requested five days later using the same two credencials no questions were asked.
So, while I agree with
@Kanga , we can rationalise the second credencial as starting the day of the first stamp.
Kia kaha (take care, be strong, get going)