Hi David,
Welcome to the Prismic community,
Before troubleshooting this issue, I would like to be sure it is not related to the preview issue that we have just deployed a fix to it today.
https://community.prismic.io/t/toolbar-preview-cookie-issue/6608/39
For that, can you please re-check if it is working now without the change that you have proposed? And would be great if you can share your repository name (in a private message if necessary).
Best,
Fares