I am having the exact same use case and issue, but both my local machine and Heroku have the exact same content yet I get the same error. And I am getting a lot of graphql schema errors.
I think the gatsby-source-strapi plugin isn’t working correctly and is not transofrming the strapi graphql schema to the one gatsby uses. Which, I don’t know why that is happening. It works when the strapi server works on my local machine.
How exactly did you fix this issue? Did you run into it again?