I have NEVER played a nightmare mission but on the ending screen of a phobos extermenation mission it stated that i had recived Fortitude, I checked my mod collection and there it was. Not sure what enemy it came from but its probably a glitch
Sometimes the system shoves you into a nightmare mission without noticing you, happened to me twice. If the nightmare malus is not very obvious (you realize you're in a nightmare mission when your shield is missing), you may not recognize that you just played a nightmare mission.
If ur in a party with others and everyone has voted and the 60 second counter has started. if one person backs up and chooses a different node regardless of vote count it will send the whole party into that node instead. THat may be another way this could happen i think they should update the game so u have to press X to play normal and press Square to go into a nightmare mission. Because i dont know about anybody else when i select a node (on PS4) i automatically hit X going for a regular game