Phone password question

So in my relationship we are fine with each other having our phone passwords because that’s one way we express trust. I’ve known my bf’s password for at least a year, maybe longer. Two days ago I saw that he had “accidentally” clicked some onlyfans links on Instagram. I talked to him about it, he said it was an accident and that he has absolutely no interest in using onlyfans. So I took his word for it and we ended the discussion saying that it’s important to trust each other and that we have no problem with the other person going on our phones because we have nothing to hide (again, that’s how we express trust). So today, it’s two days after that conversation and I noticed his phone password is changed. I find that very odd that he just changed his password after we had a conversation about trust and being totally fine with each other knowing our phone passwords.

I’m trying to find a way to bring this up to him because it feels kinda sketchy for him to change his password if he supposedly has nothing to hide.

Background: we’re in our mid-20’s and have been together almost 2 years.

Note: every relationship has different rules when it comes to phone passwords but in OUR relationship we feel like knowing each other’s passwords gives the other person peace of mind. Not for the sake of snooping, just for the sake of being trusted with having the password.

Any advice on how to ask him about why the sudden change in password would be appreciated.

Hateful comments (ex. just breakup, you don’t trust him, etc.) are not welcomed.