The "fortnight (meaning 2-week period) bug" is a bug that is not associated with some shit game and affects all iOS devices attemtping to upgrade, downgrade or re-restore to an unsigned firmware lower than iOS 12.2 using Futurerestore.
Before this bug was "introduced", whem attempting to upgrade, downgrade, or re-restore to an unsigned firmware using Futurerestore, 1 out of 2 things would occurs:
- The SEP is compatible with the unsigned firmware, and the restore succeeds, or...
- The SEP is incompatible with the unsigned firmware, and the restore fails entirely.
Now that the bug happens, the restore can succeed, but your device will stop working after 2 weeks if you have a passcode.
Many consider this bug as a "bootloop", but note that this is only from a practical standpoint, as you're still able to boot your device, but you have around 8-10 seconds (which is nothing) before the device turns off, making it impossible for you to use it again.
There are a few workarounds, but they aren't ideal unless you really want to keep your firmware. One of them invloves making a backup with iTunes or iCloud, removing your jailbreak, then calling on mobile_obliteration (aka "Reset all Content and Settings"), or iCloud erasing your device. This will give you another 2 weeks, and you'll constantly have to do this as the bug will still occur unless you don't set a passcode
Alternatively, you can upgrade to iOS 12.2 using blobs, as iOS 12.4.x's SEP and Baseband are fully compatible, and won't cause this bug. This ultimately gets rid of the bug, while still being on a jailbreakable firmware.