I too am having the same issue. No modifications whatsoever on the invoices yet prompting to save changes - this is not how it used to work. 

We lock changes for users on invoices issued on prior months but our team are still able to preview and print said invoices before this change. Now, it's prompting them for a password just to print and preview because Quickbooks assumes a change has been made, when there is none.

It applies on all invoices, old or new, asking to save changes when no changes have been made, or no changes have been attempted to be made.

I believe this is a bug that needs to be resolved. Previous response of "It means if you tried to make some changes on the invoice but at the end, the invoice wasn't changed. It will still alert you if you'd want to leave the invoice without saving." is not the answer as opening an invoice to view is by no means trying to make any changes.

Please advise on any immediate resolution. Thanks.