Updating games usually fixes bugs, but every now and again some new ones creep in, and Smash Bros. Ultimate’s 2.0.0 update sadly falls into the latter camp.
Some players have noted that playing as Piranha Plant in All-Star mode, Duck Hunt in Century Smash, or Mii Swordfighter in Century Smash, can cause the game to crash and corrupt your save data.
If you’ve been affected, you may still be in luck if you’re subscribed to Nintendo Switch Online, as you can restore your cloud save.
Hopefully Nintendo will issue a quick fix soon, but until then, maybe steer clear of those modes for now. No one wants to lose all their spirits, after all!