It's the frfdate cookie. It's set to expire at the end of a session. The skip_frf_cookie is properly persistent.
I moved over to my desktop and used Firefox User Agent Switcher to emulate a mobile host because it is easier for me to examine cookies.
Note the expiration field.
On my mobile devices, I actually do quit out of the browsers. I just don't home-screen out of them. On iOS, if any tabs have non-blank windows, it will stay active, and a single blank tab will quit. For Firefox, I actually hit 'quit'. If you want to duplicate, try rebooting your device. Once a session quits, that cookie goes away, and a re-visit to TCF brings up the nag screen.
I simulated it by both quitting and re-starting my desktop browser (masquerading as iOS Safari), and simulating a quit by deleting the frfdate cookie. Both caused a nag prompt revisiting TCF.