I understand this can be concerning, especially for those invoices that shouldn't be subject to taxes.
 

You can try pulling up the Audit Log to see any history changes made in your invoices. Here's how:
 

1. Navigate to the Gear icon.
2. Under Tools, select Audit Log.
3. Filter the Users, Events, or the Date changed. 
4. Click View to see the history.
    

If you have verified that there are no changes made. I recommend reaching out to our Live Support team again to follow up on your previous case and check if there are any updates. Here's how:
 

1. Go to the Help icon in the upper right corner of your screen.
2. In the Search bar, type Contact Support to display the option.
3. Select the Contact Support option.
4. Choose your preferred method of communication.
    

Also, here are our support hours:
 

Solopreneur, Simple Start, Essentials, Plus
 

• M-F 6 AM to 6 PM PT
• Saturday, 6 AM to 3 PM PT
   

Advanced
 

• any time, any day.
   

Let us know in the comments below if you have any other questions about QuickBooks.