EPiServerForms.js expects an ajax response in camelCase, but JsonResult renders the response in PascalCase. Therefore, when using the JS submission mode for forms, the client-side script misinterprets a true value for isSuccessful in the response as a failure. Though the form has been submitted successfully and the data saved and visible on the list of submissions, the user cannot tell that their submssion succeeded because no message is displayed.
Configuring JsonOptions in Startup.cs seems to fix this:
services.Configure<JsonOptions>(o =>
{
o.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
});However, I'm not sure what else this might break.