I think this is covered here:
The reason the app locks when you close it, is the reference to your decryption key (in memory only) is removed when the app is closed, requiring the use of biometrics to get back to that key and unlock your vault 
I think this is covered here:
The reason the app locks when you close it, is the reference to your decryption key (in memory only) is removed when the app is closed, requiring the use of biometrics to get back to that key and unlock your vault