The problem was that audience
was set when requesting access tokens from the react-admin, while I did not include this in the swift login implementation.
Decoding the JWT on jwt.io and the following thread lead to this conclusion.
https://community.auth0.com/t/access-token-too-short-jwt-malformed/9169/11?u=kennethphough
Adding the audience in the following code resulted in the correct jwt being returned and successful access to backend.
Auth0
.authentication()
.login(
usernameOrEmail: self.email,
password: self.password,
realm: "Username-Password-Authentication",
audience: "<YOUR_AUDIENCE>", // <- This is what I forgot
scope: "openid profile email"
)