Lockdown only locks the device and disables fingerprint as an unlock method for the next lock. Data is still not at rest and remains in AFU. If you're using a user profile then the End Session button (GrapheneOS feature) purges keys and puts that specific user profile back at rest / in a BFU state!

It's just a frill to prevent someone unlocking your phone with your finger while you sleep or something, it doesn't prevent much, may be possible to rename or remove this upstream feature to avoid misleading people. Chain of custody and device seizure processes typically instruct the device needs to be contained in a faraday bag as soon as possible and sent to a lab handled by a professional. Keeping a phone in the open exposed to any networks is very bad for them as it means someone could erase the device remotely.

Labs can come in different degrees of capability and they can be moved to better labs if the current lab fails to extract evidence. Good example is a local PD to an FBI lab. Some may be instructed to go directly to higher levels. I'm aware the FBI has a few national labs called Regional Computer Forensics Laboratory (RCFL) who get involved with serious crime seizures or more advanced/risky seizure targets. They do some pretty weird stuff!

By that time the phone moves to a lab the auto-reboot should have triggered unless you're so high priority they make the move to a lab and ask for that fingerprint on the same day. If they are aware of the nature of the device, they could try getting to work immediately in a portable lab as well which changes the circumstances... but they would need to know you're using GrapheneOS. FBI do a lot of surveillance work so they'd watch you to make sure they can figure out your PIN. Using a fingerprint protects this from happening.

The duress PIN / password is in the works, and also work towards a second-factor PIN for fingerprint unlocks have been quite steady and is heading towards the late stage. It's likely to combine these features too. While they are good benefits, it also means that they will treat the device differently if they know this feature is present on your phone before they get it. They won't make you touch the device at all, but with auto-reboot that could also be a blessing.