Create React App not showing error message with ErrorBoundary

As per this issue on github,

On page refresh, I see Sorry, an error occurred for, literally, a second then the full error stack displaying to the user.

@DanAbramov has made it clear that

This is intentional. An unexpected error is still an error. (We don’t recommend using error boundaries for expected errors or control flow.)

Error boundaries are primarily useful for production, but in development we want to make errors as highly visible as possible.

Also The error visible is just an overlay and your ErrorBoundary message gets hidden behind the Error overlay

To check if the Error is actually present, you can inspect element and delete the overlay from DOM, and you would be able to see the error message

Check this CodeSandbox:

The full error stack is just an overlay that's shown when you run the application in development mode. It won't get shown in production. (You can close it by clicking the 'X' button in the top-right corner.)