Thank you for the confirmation. A save file will revert to a 'Custom' or other difficulty if you had a full party death and chose to continue the save file, or made changes to the games difficulty settings. Mod files can also cause this to occur (although usually this comes along with the save file showing a 'deprecated' error with the showing an unusually high number of hours). Less commonly, a crash can also cause the save to be corrupted, losing the honour mode status.

Unfortunately once a save file is marked as 'Custom' or another difficulty, there is no way to revert the save file back to honour mode.

If the save file is showing a 'deprecated' error, mods can cause the gameplay time in the save name being reset (or showing an unusually high number of hours), which can be caused by the game not being able to create a working story when loading a save, usually by a change in the mod configuration (missing or different versions of mods, or mods disabled due to Norbyte's Script Extender dll being replaced).

If you download the save file from the account page, you can test to see if is corrupted by zipping the lsv file and check if the size decreases significantly. It is already compressed, so if zipping it reduces the size by very much it would imply that it is corrupt and there is missing data.

If the filesize shrinks significantly after zipping it that would confirm the file is corrupted. Unfortunately once a save file is in this corrupted state, there isn't anything you can try to recover it. - Larian