Symmetric keys are not returned on the JWKS endpoint, as they don't have a public key. Per the docs this api:
returns public keys generated by FusionAuth, used to cryptographically verify JWTs using the JSON Web Key format
If you create an RSA or EC key which is an asymmetric key pair - the public key will be returned on the JWKS endpoint. If you don’t have any key pairs configured , it will be empty. Out of the box, you’ll only have one HMAC key which we don’t publish in JWKS.