Super Smash Bros. Ultimate players report corrupted save files when using Piranha Plant
You might want to hold off using Piranha Plant in Super Smash Bros. Ultimate All-Star Mode for the time being.
Nintendo deployed the latest update for Super Smash Bros. Ultimate overnight, which included DLC character Piranha Plant. Since applying the update, many players have reported using the character causes their save files to become corrupted when used in All-Star Mode.
Various users have reported the issue online, and as Nintendo Life points out, SSBU player Xoana lost 130 hours' worth of save data.
Xoana goes on to say data was backed up to the cloud due to having an online subscription; however, the original data wasn't saved. This means their World of Light progress and Spirit collection also disappeared.
Hopefully, if you are experiencing save data corruption, you have an older save file in the cloud to revert to. But, you need to be a subscriber to Nintendo Switch Online. And hopefully, you backed the game up recently.
It's not just Piranha Plant though, as other SSBU players are reporting issues in Century Smash issues when using Duck Hunt and Mii Swordfighter. Playing as one of the characters in this mode could also cause save data to be corrupted (thanks, reddit).
Not all users are experiencing issues, but you may want to hold off playing until a fix is issued by Nintendo. It's always better to be safe than sorry.