Difficulty change bug?

So I randomly went into the game settings menu today, and noticed that my inquisition was set to normal mode. I had set it to nightmare mode at the start of the game, so I wondered if it glitched out. My current saves span 70-77 hours of playtime, they were all also on normal mode. I had one save from 33 hours and sure enough, it was on nightmare. Having never even gone into the menu where changing difficulty is an option, I have no idea how it got changed... and now I have to either replay the last 40 hours of game time, or finish the game without getting the inquisitor achievement. Neither is attractive to me. Why did this happen, can it be fixed somehow?