I started an Amazons Penthesilea campaign on H/H. Everything was going fine, did the first three steps in my epic mission chain and mission 4 is to sack Sestos, (owned in this play-through by Chersonesos). Well the turn afterwards, the settlement Sestos was occupied by my defensive ally Hector of Troy which resulted in a notification telling me the epic mission was no longer available. No further missions have appeared since. Can post screen caps etc if requested.