Hello miss147,
Since that only occurs while using a particular app, you'll want to use the steps below to see if you have a Focus set up to turn on automatically while using that app:
You can schedule a Focus to turn on at certain times, when you’re at a particular location, or when you open a specific app.
1. Go to Settings
> Focus, then tap the Focus you want to schedule.
2. To have this Focus turn on automatically based on cues like your location or app usage, tap Smart Activation, turn on Smart Activation, then tap
at the top left.
3. Tap Add Schedule, then set the times, a location, or an app you want to activate this Focus.
You can also find those steps in this resource: Turn on or schedule a Focus on iPhone - Apple Support
If it appears that behavior isn't related to a Focus, the additional steps below can help address unexpected behaviors from a specific app: If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support
1. Close and reopen the app
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.
Should that still persist, you'll want to reach out to Facebook: Facebook Help Center They'll be in the best position to assist with troubleshooting their particular app further.
Take care.