I believe this is the same issue as
QTBUG-83729, which should have been resolved after 5.15.0, but I am still able to reproduce it on QtWebEngine 5.15.1.
I've loaded up a fresh Falkon install and verified the behavior from there. I've also confirmed that the issue continues to reproduce after upgrading all system packages and rebooting my machine.
See attached file for a demonstration video: