Symbolism, Imagery, Allegory
The fact that Joan's heart doesn't burn can be seen as symbolic. The undamaged heart could represent her eternal spirit, which doesn't die along with her body. Her memory lives on to inspire many more generations to come. The play recognizes this in the epilogue, when the Gentleman comes from the future to tell us that Joan has been made a saint. Like Christ, Joan is resurrected in a sense by her canonization. Of course, this is a Shavian play, so no triumph comes without a twist of irony. The epilogue hypothesizes that, if she did come back to life literally, as is said of Jesus, that she would just be burnt all over again. Perhaps, the spirit that her unburned heart symbolizes is all the world ever really needed or wanted from her.