I've been reading this on Twitter. Seems to be affecting all platforms so worth a read to those playing.
In Metal Gear Solid V: The Phantom Pain, you might want to tread carefully on two of the game's missions, as utilising Quiet as a buddy during these specific parts of the game can corrupt your save data and render it completely unusable. You'll want to avoid using her in Mission 29 and 42.
"In Metal Gear Solid V: The Phantom Pain which was released on September 1st 2015, there is a possibility that save data may get corrupted and becomes unusable if having Quiet as a buddy while playing "Mission 29" or "Mission 42"," Konami has warned. "This situation can be avoided by not using Quiet as a buddy while playing either "Mission 29" or "Mission 42".
"We deeply apologise, and appreciate your patience for further announcements regarding this issue," the publisher added. This potential save corruption issue affects all platforms, so avoid using Quiet as a buddy in Mission 29 and Mission 42, and you should be fine.