I would suggest three things:
1) Make sure you have the latest version of the app. 6.18.0 is being released and should or will soon be available.
2) Reset app cache. On iOS, you can find that in your phone settings: https://community.canvaslms.com/t5/Canvas-Student-iOS-Guide/What-Student-app-options-can-I-manage-fr...
- From your phone’s settings, select Apps
- Locate the Canvas app and select it
- Press storage and select the delete or clean button to clear data and clear the cache
3) Uninstall and reinstall the app.