The description of a Missionary shows the original religion's name, rather than the name given to religion by the player. Here I named a religion Toraism, and the missionary shows "Missionary - Buddhism". The screenshots showing the missionary are from the perspective of a second player (hotseat), but it also shows that name for me. The same happens for the prophet (screenshot from later in the game, my perspective). This is the latest game version from Steam. That's rather an insignificant bug, though it might cause confusion in some multiplayer games where players name their religions.