PDF invoice changing without being edited

I have created and use a invoice and have been using it for some time. But from time to time the PDF changes without anyone editing it and need to be reset.

Is there anything that can be done about it?

I can see that you have created a custom invoice in your PDF Editor instead of enabling the Invoice option inside the product list:

Please let us know what is changing on the PDF.

Have you tried the Invoice option on the product list field:

Please review and let us know.