When managing local users, user names or login names get changed seemingly random without an administrator making explicit changes, or incorrect names get displayed in the user editing interface.
This is likely caused by the autocomplete/autofill feature of your respective web browser. Despite following best practices, certain text input fields will be populated with previously entered data (commonly names and address data) by your browser automatically. When this happens, the input fields are commonly color-coded with a blue background to indicate they have been autofilled. Unless you save the changes to a user after them being autocompleted, this can be ignored. If a user got unintentionally changed, it will need to be renamed back to its previous name.
To avoid autocomplete from creating the opportunity for your data to be corrupted, we highly recommend to use automated user provisioning systems like SCIM or LDAP. In case this is not possible, disable the autofill/autocomplete feature in your browser by going into your browser settings.