invalid session in undertow using basic auth and stateless sessions - spring-boot

Question

I have a grails 4 (based on spring boot) application that is experiencing sporadic failures. At this point I cannot find anything even pointing to what the problem may be, and hope that perhaps someone can assist.
The scenario where the error occurs is one application making a REST call to another. These are all made with a common username and password provided via basic authentication. At some point after the application has been running successfully (> 12 hours in most cases seen so far) these calls begin to fail with this error:
org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.IllegalStateException: UT000010: Session is invalid XVBnzVaxVhijSIz2xk_Tl5fqsJD9n30ggd_614iF
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1014)
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898)
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)
at io.undertow.servlet.handlers.ServletHandler.handleRequest(ServletHandler.java:74)
at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:129)
at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:209)
at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:178)
at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:61)
at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:131)
at io.undertow.servlet.handlers.FilterHandler.handleRequest(FilterHandler.java:84)
at io.undertow.servlet.handlers.security.ServletSecurityRoleHandler.handleRequest(ServletSecurityRoleHandler.java:62)
at io.undertow.servlet.handlers.ServletChain$1.handleRequest(ServletChain.java:68)
at io.undertow.servlet.handlers.ServletDispatchingHandler.handleRequest(ServletDispatchingHandler.java:36)
at io.undertow.servlet.handlers.RedirectDirHandler.handleRequest(RedirectDirHandler.java:68)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43)
at io.undertow.servlet.handlers.ServletInitialHandler.dispatchRequest(ServletInitialHandler.java:251)
at io.undertow.servlet.handlers.ServletInitialHandler.dispatchToPath(ServletInitialHandler.java:186)
at io.undertow.servlet.spec.RequestDispatcherImpl.error(RequestDispatcherImpl.java:501)
at io.undertow.servlet.spec.RequestDispatcherImpl.error(RequestDispatcherImpl.java:427)
at io.undertow.servlet.handlers.ServletInitialHandler.handleFirstRequest(ServletInitialHandler.java:308)
at io.undertow.servlet.handlers.ServletInitialHandler.access$100(ServletInitialHandler.java:78)
at io.undertow.servlet.handlers.ServletInitialHandler$2.call(ServletInitialHandler.java:133)
at io.undertow.servlet.handlers.ServletInitialHandler$2.call(ServletInitialHandler.java:130)
at io.undertow.servlet.core.ServletRequestContextThreadSetupAction$1.call(ServletRequestContextThreadSetupAction.java:48)
at io.undertow.servlet.core.ContextClassLoaderSetupAction$1.call(ContextClassLoaderSetupAction.java:43)
at io.undertow.servlet.handlers.ServletInitialHandler.dispatchRequest(ServletInitialHandler.java:249)
at io.undertow.servlet.handlers.ServletInitialHandler.access$000(ServletInitialHandler.java:78)
at io.undertow.servlet.handlers.ServletInitialHandler$1.handleRequest(ServletInitialHandler.java:99)
at io.undertow.server.Connectors.executeRootHandler(Connectors.java:376)
at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:830)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.IllegalStateException: UT000010: Session is invalid XVBnzVaxVhijSIz2xk_Tl5fqsJD9n30ggd_614iF
at io.undertow.server.session.InMemorySessionManager$SessionImpl.getAttribute(InMemorySessionManager.java:512)
at io.undertow.servlet.spec.HttpSessionImpl.getAttribute(HttpSessionImpl.java:122)
at org.springframework.web.servlet.support.SessionFlashMapManager.retrieveFlashMaps(SessionFlashMapManager.java:49)
at org.springframework.web.servlet.support.AbstractFlashMapManager.retrieveAndUpdate(AbstractFlashMapManager.java:94)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:934)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006)
Calls to the same endpoint with the same basic authentication from Postman succeed.
For these calls, security is configured as such:
http
.headers().frameOptions().sameOrigin().and()
.csrf().disable()
.requestMatcher(basicAuthMatcher)
.authorizeRequests()
.anyRequest().authenticated()
.and()
.httpBasic()
http.sessionManagement().sessionCreationPolicy(SessionCreationPolicy.STATELESS)
The parts that are suspicious to me are that the stack trace is coming out of SessionFlashMapManager. To my understanding there should be no session. There certainly is none that I intend to use.
Has anyone seen errors like this before, or have suggestions for how to diagnose it?