The law is probably related to anti-money-laundering, sanctions compliance, etc. If they detect something possibly criminal, it's illegal for them to "tip off"

The law is probably related to anti-money-laundering, sanctions compliance, etc. If they detect something possibly criminal, it's illegal for them to "tip off" the owner of the account. This is true of basically any financial institution. Of course, it is a problem that Monzo had a bug that caused your account to get flagged for this.