After further investigation, there were configuration issues with the environment used to log this bug. After fixing those issues, this was not reproducible in a properly deployed Portal environment.
対処法
Workaround 1: Federate ArcGIS for Server with Portal using port 6443 or to a Web Adaptor behind the NLB.
Workaround 2: Remove the HA Portal and use only one Portal which allows federation through the NLB.