Customer Emails Are Overwritten

[Sass]

I'm using the default Prestashop theme on 1.7. When I open a customer record, the previously used client's email address is entered into the newly opened client record. 

I have to copy the client's email address BEFORE opening their record in order that I can reenter and save their details. As I am setting up targeted new clients, I currently also have their passwords, but this won't always be the case. Therefore I am very concerned that I won't be able to safely amend groups when clients proactively register to my site.

Why is Prestashop not saving email addresses as fixed data? How can I stop it putting in an incorrectemail address whenever I switch the client record into Edit mode?

Context: This is a trade site - so I want Visitors to view the site without prices. When they register as Customers, I want to verify their details and upgrade them to a Guest. Guest settings give them access to pricing and ordering.