In development environments running a containerized local Keycloak server, or in some more complex network topologies, not all clients will be able to access the Keycloak server at the same URL. However, token validation in the Node.JS adapter middleware only allows tokens with an issuer exactly equal to the realm URL.
If a web browser obtains a token at a realm URL accessible to the developer machine/VM, and passes it as a bearer token to a back end service which must access the Keycloak server at different URL (e.g. a hostname on an internal container network), the token will fail validation.
Some other JWT validation libraries allow validation against multiple tokens (in an array). This would be a big improvement to the Keycloak middleware.