Gatsby Cloud has a limit of 20MB for page-data.json. In most cases, the 503 error on the JSON file typically means that the JSON file is too big and it's above the Gatsby cloud limit. To check the size for page-data.json, run gatsby build using Gatsby CLI tool and search for the file in the public folder.
We recommend limiting or filtering your graphql template query to fetch less data. You'll find more information here.