I am using Gatsby and Prismic and seeing inconsistent results with previews. Rich text fields are also not being run through the HTML Serializer that is configured in gatsby-source-prismic in gatsby-config
I'm Angelo, the developer of the Gatsby plugins. That error appears when your Custom Type JSON schemas are out of sync with what is saved in Prismic (assuming there isn't a bug in the plugin).
Do you use the customTypesApiToken plugin option to automatically pull the Custom Type JSON files? Instructions can be found here: Install the Gatsby Source Plugin - Prismic
Regarding the HTML Serializer issue, this could be a result of the above error (the Custom Type JSON file could be outdated so it doesn't know that your Rich Text field is a Rich Text field).