Checkout: Last Name input writes into First Name for problem_user

Environment: https://www.saucedemo.com, account problem_user, Chromium 153 (Playwright 1.63.0), Windows 11. Observed 2026-09-15.
Severity: Major
Priority: P2

Steps to reproduce:
1. Open https://www.saucedemo.com and log in as problem_user.
2. Click "Add to cart" on Sauce Labs Backpack.
3. Open the cart and click "Checkout".
4. Type "Ada" in First Name.
5. Type "Lovelace" in Last Name.
6. Type "10115" in Zip/Postal Code and click "Continue".

Expected result: First Name shows "Ada", Last Name shows "Lovelace", and Continue opens "Checkout: Overview".

Actual result: Each character typed in Last Name replaces the First Name value, so First Name shows "e" and Last Name stays empty. Continue shows "Error: Last Name is required" and checkout cannot continue. Pasting the whole value gives First Name "Lovelace" instead.

Reproducible: 6 of 6 attempts (3 typed, 3 pasted). standard_user is not affected.

Impact: Any customer on this account cannot complete checkout, so every order is blocked. There is no workaround in the UI.

Evidence: screenshot problem-user-last-name.png and Playwright trace trace.zip attached.