The JwtDecoders.fromIssuerLocation will attempt to resolve the jwks_uri from the OpenID Connect discovery document found using the issuer URI.
https://github.com/spring-projects/spring-security/blob/848bd448374156020210c329b886fca010a5f710/oauth2/oauth2-jose/src/main/java/org/springframework/security/oauth2/jwt/JwtDecoders.java#L119
The FusionAuth JSON Web Key Set (JWKS) only publishes the public key from asymmetric key pairs. This means there are no public keys published and the Spring boot library cannot verify the token signature.
For example, if your issuerUri is https://example.com then the OpenID Discovery URL is https://example.com/.well-known/openid-configuration and the value for jwks_uri found in the JSON response from that URL will be https://example.com/.well-known/jwks.json. If you hit that URL you will see no public keys are being returned, this is the JSON that the library is consuming in an attempt to build the public key necessary to validate the JWT signature.
To use this strategy then you'll need to configure FusionAuth to sign the JWT using an RSA or ECDSA key pair instead of the default HMAC key which is symmetric.
Generate a new RSA or ECDA key pair in Key Master (Settings > Key Master) and then ensure you have your JWT signing configuration use that key. The primary JWT signing configuration will be found in the tenant, with optional application level overrides.
https://fusionauth.io/docs/v1/tech/core-concepts/tenants/#jwt
https://fusionauth.io/docs/v1/tech/core-concepts/applications/#jwt