Check your response. Are you certain that the user is successfully registered every time? It could be that the generated password does not adhere to the validation rules or that the email is not unique (if you are not using the RefreshDatabase trait)?
For this test, why not manually define the email and password, e.g. "[email protected]" and "testpassword1234"?
I didn't put unique validation here, but this is not the problem because if I didn't put the RefreshDatabase trait, I would have a different error. Also, the error had to be every time, not once in 3-4 tests.