https://www.gamespot.com/articles/nintendo-fixing-switchs-playtime-glitch-with-an-up/1100-6457156/

This past weekend marked the first anniversary of the Nintendo Switch, but the celebration was dampened for many Switch owners when they discovered the system had begun seemingly resetting their play times. Fortunately, the issue appears to be temporary, and Nintendo has announced an update is on the way to resolve it.

"Some Nintendo Switch owners' play activity info is displaying incorrectly, but play activity is still being recorded accurately in the background," Nintendo said on Twitter. "A future system update will resolve this issue [and] ensure play time data is correctly displayed. We apologize for the inconvenience."