https://www.gamespot.com/articles/nintendo-switch-is-apparently-resetting-play-times/1100-6457122/

UPDATE 3: Nintendo has shared more details about the Switch playtime glitch. The company says that users' Switch play activity "is still being recorded accurately in the background" despite not displaying correctly for certain titles that were first played a year ago. Nintendo has also announced that it will deploy an update that resolves the issue.

UPDATE 2: While many more Switch owners are noticing their play times apparently being reset on the console, it appears this, thankfully, isn't permanent. Nintendo fan publication NF Magazine confirmed on Twitter that your play activity isn't deleted after one year, but rather disappears for 10 days--the length of time it normally takes for it to show up in your profile after you first begin playing a title. After the eleventh day, it reappears as normal. Nintendo has acknowledged that some Switch owners' play times have begun "displaying incorrectly" on the console's first anniversary but thus far has only said it will have more details to share about the issue "in the near future."