As previously reported, the PC version of the otherwise excellent expansion was marred by a save data error which could effectively delete player's characters. Capcom acknowledged this issue in a Steam blog post, confirming that "save data would not convert to the new file format if the save data and the game hadn't been updated after October 30, 2018, the day when Kulve Taroth had been added to the game via Title Update." In other words, if you hadn't played since October 2018, your save wouldn't work.
I'd love to joke that this error illustrates the power of Kulve Taroth, but this really is no laughing matter. Monster Hunter World's Steam page was flooded with countless reports of players losing dozens to hundreds of hours of play time, and I can imagine how disheartening that must be, especially going into a new expansion. The good news is that those saves aren't necessarily gone. Once the new patch is out, it's possible that the game will begin recognizing older saves.
That said, there's no guarantee lost saves will return, either. Capcom didn't promise anything, and its blog post only advises players to avoid the issue by shutting down the game if they see the in-game prompt, "Failed to read saved data." Here's hoping that players who selected "Yes" when this prompt asked them to "Create new save data" will still be able to access their old save via a different slot once this week's patch is released.
Lost saves aside, Capcom also confirmed that this update will tackle the CPU issues many players have experienced following Iceborne's release. Monster Hunter World has always been a CPU-heavy game, and it wasn't terribly well-optimized at launch, but it's gotten much better in the past year. Iceborne's poor performance was definitely an outlier, so hopefully it's at least brought up to the level of the rest of the game.
Check out the latest Monster Hunter World: Iceborne dev diary for a look at what's ahead for the game.