Last Name cannot be entered during checkout for "problem_user"

[pramam]

When logged in as "problem_user" the checkout page does not accept an input for Last Name. But once you type in postal code, it accepts a Last Name which does not overwrite the First Name.

Steps to reproduce:

• Log in as "problem_user" (Password: "secret_sauce")

• Add an inventory item to the cart

• Click on cart icon on top right

• You are now on a page that shows the one item

• Click on CHECKOUT

• Enter First Name 'John' (please scroll down, more text below the image)
  

• Enter Last Name 'D'
  Bug: The first letter of the Last Name overwrites the first name
  When you type 'Doe', FirstName is replaced with 'D', then 'o', then 'e'.
  

• But once you type a postal code, it accepts an input for Last Name that does not overwrite First Name (This step is hard to reproduce sometimes):