Mobile applications normally collect cache files that help to save you time while browsing. Sometimes, these files can become outdated or corrupted, causing issues like you're experiencing with the invoice numbers. Clearing the app's data can fix it. This can be done on the app or on your phone settings.
For QuickBooks Online app:
Android - Select the menu at the top right and choose Settings. Then select Refresh Data and choose Yes.
iOS - Select the menu at the top right and then choose Help & Feedback. Then select RefreshData and Refresh.
In your Phone Settings:
Android - From your phone's settings, select Apps (varies by device). Choose the QuickBooks app from the list and select Storage. Select clear data.
iOS - From your phone's settings, select Settings, then, click on General and choose iPhone Storage. Select an app that's taking up a lot of space. To free up the storage without deleting the documents and data for the app, select Offload App.
You can refer to this article for entire details of the troubleshooting. You may notice that the article may be for United States but is applicable to global versions like India: Mobile App's troubleshooting.