@mrspock When you enable the Unlock with PIN option, there is a pre-checked option to enable Lock with master password on browser restart, which forces the behavior that you are currently experiencing. Previously, you must have cleared that checkbox, thus disabling the Lock with master password on browser restart option (and allowing you to unlock with a PIN on restart).
Based on the various recent threads on this and similar topics, I believe that some recent update may have caused the Lock with master password on browser restart option to become reset to its default setting (enabled). The solution would be to disable the PIN, then re-enable the PIN, and ensure that you clear the Lock with master password on browser restart checkbox.
P.S. On the desktop client app, the option is called Lock with master password on restart. On the mobile apps, there is a dialog box that asks if you want to require unlocking with your master password when the application is restarted.