I went back into the Nameless Knight's Tomb, only to encounter another bug while fighting the Disloyal Knights.
They struck hard, and overcame me, but rather than my being transported back to Caliburn I remained in the battle circle in the defeated state, alone (well, except for Duke Ellie, my pet). Every round, I suddenly had a small number of hit points again, and they attacked normally even though I was still displayed as defeated.
Then things got stranger when I gained a power pip, still defeated at the time. And the round after that, I was suddenly undefeated and able to cast again.