This bug is a relatively new problem for jailbreakers as previously, when you futurerestored, one of two things would happen:
-
the SEP was compatible, and the restore succeeds, or
-
incompatible, and the restore fails
The new, third option: the restore succeeds, but after two weeks (a fortnight, hence the name), if the device has a passcode set, the device will... "kinda bootloop". What happens is the device will fully boot to the lock screen, but when you enter the passcode, the device reboots, so from a technical standpoint, it's not a bootloop, but from a practical standpoint... it's a bootloop.
Currently the only way to workaround this bug is to make an iTunes or iCloud backup on day 13, restore rootfs, and then call mobile_obliteration
(the process that runs when you erase all content and settings or iCloud erase your device). After you erase the device, you have two more weeks before you have to repeat the process.