Two deployments configure discordant values for externalDocs, i.e.: one of them has it defined by a static openapi.yaml file, while the other uses a OASModelReader custom instance to initialize the OpenAPI model with a custom value for externalDocs and also deploys a REST resource class which is annotated with @ExternalDocumentation.
This results in the final OpenAPI documentation to have an empty externalDocs element, because of discordant values.

According to the feature implementation documentation, such conflict should be resolved by sourcing the value from properties set in the microprofile-config-smallrye subsystem, but this does not happen, despite the following server configuration seems to be valid at runtime:

<subsystem xmlns="urn:wildfly:microprofile-config-smallrye:2.0">
            <config-source name="props">
                <property name="mp.openapi.extensions.server.default-server.host.default-local.externalDocs.url" value="http://system-property-based-local-service-router-external-docs.org"/>
                <property name="mp.openapi.extensions.server.default-server.host.default-local.externalDocs.description" value="System property based Local Service Router external documentation"/>
            </config-source>
        </subsystem>