AbstractImmutableWebFailoverTestCase.testFailover(...) includes logic to validate the behavior of "immutable" session attributes. It does this by configuring a mutable object to be immutable (e.g. via distributable-web.xml), invoking a request on the primary owner that mutates the value, then invoking a subsequent request on a another server and validating the the mutated value is not visible.
However, if the initial request that "mutates" the object is received before the request creating the session has complete, the cache operation will be deferred until the second request completes. The result is that the subsequent request to a non-primary owner returns the correct value, whereas the test assumes this will read the outdated value.

As seen here: https://ci.wildfly.org/buildConfiguration/WF_PullRequest_WildFlyPreviewLinuxJdk21/537161?buildTab=overview#%2Ftestsuite%2Fintegration%2Fclustering%2Ftarget%2Fwildfly-1%2Fstandalone%2Flog;%2Ftestsuite%2Fintegration%2Fclustering%2Ftarget%2Fwildfly-1%2Fstandalone;%2Ftestsuite%2Fintegration%2Fclustering%2Ftarget%2Fwildfly-2%2Fstandalone%2Flog;%2Ftestsuite%2Fintegration%2Fclustering%2Ftarget%2Fwildfly-2%2Fstandalone;%2Ftestsuite%2Fintegration%2Fclustering%2Ftarget%2Fwildfly-3%2Fstandalone