@org.springframework.boot.autoconfigure.AutoConfiguration(after = WebMvcAutoConfiguration.class)
@org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)
@org.springframework.context.annotation.Import(GrailsViewResolverAutoConfiguration.RemoveDefaultViewResolverRegistrar.class)
public class GrailsViewResolverAutoConfiguration
extends java.lang.Object

Without the auto-injected @EnableWebMvc, Spring Boot's WebMvcAutoConfiguration is active and contributes a defaultViewResolver (InternalResourceViewResolver). For a Grails application that does not use JSP/InternalResource views (e.g. a REST/JSON-views app) this catch-all resolver resolves any unmatched view name to a servlet forward, producing Circular view path [index]: would dispatch back to the current handler URL errors. This is the single place the defaultViewResolver is removed for every Grails servlet web application (GSP, JSON/Markup or plain), so Grails' own view resolution is used instead. grails-gsp only loads for GSP applications, so the removal cannot live there. Ordered after WebMvcAutoConfiguration so the bean exists by the time the registrar runs.

Disable with grails.web.removeDefaultViewResolverBean=false. The earlier spring.gsp.removeDefaultViewResolverBean (when removal lived in grails-gsp) is still honoured for backward compatibility, but is deprecated.