Not super familiar with PIN unlock since my mobile device integrates biometrics for unlock, possibly when you set up PIN unlock it was set to require your master password when the app is closed and restarted.
This can be changed to always allow PIN unlock if you choose.
- A dialog box will appear asking whether you want to require unlocking with your master password when the application is restarted. Tap Yes to require your Master Password instead of PIN when the app restarts. Tap No for the ability to unlock with the PIN when the app restarts.