Hi Flotsam49,
Welcome to Apple Support Communities. Since it sounds like this happens with the Fitbit app specifically, it may be best to reach out to the developer directly. If you need help locating contact information for the developer, use the following steps from How to contact an app developer - Apple Support.
"Contact the developer of an iOS, iPadOS, or watchOS app
- Open the App Store on your iPhone or iPad.
- Search for the app, then tap it. If you can't find it, tap the account button
or your photo at the top of the screen, tap Purchased, tap "Not on this [device]," then tap the app. - The app's product page appears. Scroll to the Ratings & Reviews section, then tap App Support. If you don't see App Support, make sure that you're signed in with the correct Apple ID."
If you believe this may be unexpected for the app you can also take these steps from If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support to resolve issues since this happens in the Fitbit app only.
"1 Force the app to close. Then open the app to see if it works as expected.
2 Restart your device
Restart your iPhone or restart your iPad. Then open the app to see if it works as expected.
3 Check for updates
To see if updates are available for the app, open the App Store, tap your profile icon, then scroll down to see if updates are available. If updates aren't available for the app, update the software on your iPhone or iPad.
4 Delete the app, then redownload it
Delete the app. When you do this, you might lose data that was stored in the app. Then redownload the app. Then check to see if the problem with the app is fixed."
All the best.