Joomla Session Timeout Messes Up Layout

-- HikaShop version -- : 3.2.1
-- Joomla version -- : 3.8.1
-- PHP version -- : 7.1.1
-- Browser(s) name and version -- : Firefox 56.0 (64-bit)

Steps to reproduce:

1. Set Global Config > System > Session Lifetime to 1 (for 1 minute).
2. Checkout Workflow is set as: Cart/Coupon -> Login -> Address -> Payment.
3. Go make a purchase (I have New Account Activation set to None, so no activation required).
4. When you get to the Address page, wait at least 1 minute for Joomla session to timeout.
5. Click OK for the address.
6. You are returned to cart page, but not really. The URL is still on the Address step. The cart page shows up seemingly in its own frame.

Attached screenshots for reference.

EDIT: This also happens on the Login page, if you timeout and then click the Login button or the Register button. I'm guessing it's any page with a form that has a button other than the Next button. Also, I edited the topic of this thread to not include "Cart", as that's specific to my workflow where "Cart" is on the first step of checkout.