In the meantime, you'll need to delete and recreate those tax exempt invoices became a journal entries.
Open the journal entry.
At the bottom, click More.
Select Yes to confirm that you want to delete the transaction.
Once done, you can recreate those tax exempt invoices
Also, for deleted transactions, you can pull up the Audit Log report to view the changes made and who made them. However, you'll need to have full access rights to run this report. You can learn more from this link: Learn about the audit log.
Please let me know if you need anything else. I'll be here to help. Enjoy your day!