I've seen ordinary people forget things they've done every day for years. It's not something that never happens.
I'm just saying the police will unlikely drop their line of questioning after you claim you forgot the PIN. They'd probably then seek to prove you did know it - eg ask where's your boarding pass is, ask if you messaged/called anyone before/during your flight, try to establish the time the phone was last used, reducing the period of forgetting etc
IANAL/officer of course