This sounds like what is currently happening to me. I have a Fitbit One, which I've been using for almost 10 years and haven't had any issues until recently. It still tracks steps, etc. just fine, and on the current day, when I sync to the app, my step totals appear just fine in the Android 11 tablet I use. However, when it's the next day, and I go back to the previous day, there is no data. This has been happening for more than 2 weeks now, with the last day of steps shown on the app being August 26, so August 27-yesterday has disappeared from the app.
My One is syncs to the tablet and adjusts the battery level, so I suspect it's not Bluetooth. I've also uninstalled & reinstalled the app, reset the One, and turned the Bluetooth on & off, doing the process multiple times but the data doesn't appear. What's weird, however, is that my One is connected to an external site where I'm competing in a fitness challenge. Some of the missing days from the app were picked up by the external site, so my guess is that this is clearly an app/connectivity issue somehow b/c my Fitbit One is still recording my steps and they're stored on the cloud/servers somewhere and only remain in the app temporarily, i.e. the day they occur, and then just disappear.
Within the past 2 weeks, I was finally forced to migrate over to Google, and while I don't know the exact date, I'm convinced that the migration has something to do with daily data disappearing from the app the following day.
