I think this is the minimal repro.
See src/repro.clj.
TL;DR: jack-in with :repl alias to get CIDER/nREPL/Portal with the
Portal middleware enabled. Eval forms in src/repro.clj including
running the test that has reify Object with hashCode method that
throws. The test passes, but the test report that Portal intercepts
seems to break rendering until you clear the Portal buffer.