We have then made a fresh installed Magento 2.3.4 of your latest checkout, tested it, and after purchase landed in an empty cart.
Something seems to be bugging in the module.
+ Option 1: Run this command php bin/magento setup:config:set --session-save=redis --session-save-redis-break-after-frontend=15
(Please take a look at this article for more information: https://devdocs.magento.com/guides/v2.3/config-guide/redis/redis-session.html)
+ Option 2: Change the value config "break_after_frontend" of Redis session on this file app/etc/env.php (attachment below)
If you have any issues, please don't hesitate to contact us.