I'm experiencing the same issue. Even when creating a new extension version and "publishing" it the code does not update in the dev store. I've been uninstalling and re-installing the app in the merchant store to get it to work, I'll try the webPixel...

I extended my app to with a UI element in the admin to trigger the graphql mutation for now. It was picky about the JSON syntax as well and this post was helpful in resolving that. Hope this helps.

@ChrisGrin I've been having this same issue and have been unable to resolve. When you say "manually calling the mutation" how are you going about making that call? Are you using Postman or some other way to send the graphql payload? Thanks in advance...