Earlier quoted context omitted.
Somewhat loosely related, but I have something similar with the iPhone browser. Where opening the browser will shortly show the last page I had open (even I carefully closed it before closing the browser). Even if it never got me into trouble, I found that annoying as s. And could potentially make problems.
FWIW, what you’re seeing is likely just the cached thumbnail that iOS creates for the app-switcher UI, not an interactive web session.
There's a way to block that entirely for "secure' apps, but iOS could be smarter about this, and cache some stripped down view or expire that cache quicker.