This is true but there's automatic restart which will automatically restart the phone to get it back into BFU state:
> Automatic Restart is a security mechanism in iOS 18.1 iPadOS 18.1 and or later that leverages the Secure Enclave to monitor device unlock events. If a device remains locked for a prolonged period, it automatically restarts, transitioning from an After First Unlock state to a Before First Unlock state. During the restart, the device purges sensitive security keys and transient data from memory.