I appreciate you bringing this up to the Community, @mandurangpm.

To resolve the unexpected behavior and missing tax option on invoices in your QuickBooks Online mobile app, you can do some app troubleshooting.

Here's how to troubleshoot your app:

For iOS devices:

1. Tap on the Settings icon and then choose General.
2. Click on iPhone Storage and choose the QuickBooks Online (QBO) app.
3. Tap on the Offload App option to refresh the app's data.

For Android devices:

1. Open your phone's Settings.
2. Select Applications.
3. Look for the QBO app.
4. Tap Clear data to reset the app to its initial state.

If the issue persists, you can uninstall and reinstall the app. This ensures that the version you'll be using is updated.

If you have any follow-up questions, feel free to click the Reply button.