Customers not logged out following transactions

-- HikaShop version -- : 4.2.3
-- Joomla version -- : 3.9.16
-- PHP version -- : 7.2

Once a customer completes a transaction they remain logged into Joomla indefinitely. How do we fix this? Normally Joomla would log out inactive users but this doesn't seem to happen with users that have been registered through Hikashop.

Hi,

No, they will be logged out automatically after their user session reaches the "session lifetime" you configured in your Joomla configuration. That's Joomla who handles the user session, not HikaShop.
If it seems that they remain logged in indefinitely, it means that you must have configured a big value in that setting.

It's set to the default — 15 minutes.

Hi,

Then it could be that you have another extension which maintains the user session open by requesting a URL in AJAX every few minutes on your website while at least a page of the website is open in a browser.
Note also that each time you display a page of your website while logged in, it resets the lifetime of the user session. So it's possible to be logged in for hours by displaying at least one page every 15 minutes.