I stand corrected.
However, we do have these folk who saw some jail time:
Daniel, Joseph, Jeremiah, John the Baptist, James, John (James and John were imprisoned and put to death by King Herod), Peter, Paul and Silas.
I am unaware of any of these prophets, who upon being imprisoned, declared that they were not prophets while in captivity. Now, I can understand where the church could say that any edict issued by a captive (or turtured or otherwise incapacited Pope) would be ineffectual.
Which Joseph is that?
The office of "Pope" is different from the office of "Prophet". While being "Pope" may at times require "prophetic" action, the job is best summed up as "teach, rule, and sanctify". While Eugenio Cardinal Pacelli can easily teach, sanctify, and even prophesy from captivity within a Nazi prison, "ruling" as Pope Pius XII would be effectively impossible. Furthermore, by resigning the office of Pope, he would deny the Nazi enemy the victory of having "imprisoned the Pope". It was a clever move that somewhat pulled the Nazis teeth.