I don’t know how the procedere would be executed, but I imagine that police could have the phone present during an interrogation and try to nlock it there (possibly by making you to look at the phone to unlock it, if the phone has been set up to unlock this way). Once unlocked, it would be sufficient to have a peek into the camera roll or messages, until the phone locks again. I don’t know about the law, but I can imagine that if a police officer had a look into your phone, even briefly, it may be held against the one who is being interrogated.
It’s like the five stages of grief but with not knowing what the consequences of voting against your interests are.