Konami is currently working on a patch for MGS 5: The Phantom Pain which will fix the save data lost due to a game-breaking bug.
The said bug completely wipes user ‘Save Data’ if they select Quiet as their buddy during Main Mission 29 and Main Mission 42.
This is what Konami wrote on MGS 5: The Phantom Pain official website:
As previously communicated, a critical bug has been identified that may halt progress and corrupt Game Save data under certain circumstances. Further Investigation has Revealed the exact trigger of the bug, so we would like to share these details to enable players to effectively avoid the Issue.
Konami has also stated that users still can Quiet as their buddy, but under certain circumstances:
The bug is triggered only under certain circumstances in Episode 29 or Episode 42, when Quiet is used as your buddy. However, you still may safely use Quiet in these missions if you use the Butterfly Emblem throughout the mission or avoid raising your Bond Level with quiet to max during the course of the Mission.
The team is currently working on a patch to fix the bug and repair any Save Data affected by the issue. Further updates will be announced soon. In the meantime, we apologize deeply for the problem and thank you for your patience as we continued work to resolve the issue.
A release date for the patch has not been disclosed, but we do know that MGS 5 game-breaking affects the game on PC, Playstation 4, Playstation 3, Xbox One, and Xbox 360.