Hi @Craig,
5 seconds test was just to check if the Enpass's default behavior of unlocking with PIN is working correctly because in 5 seconds the chances of being killed by OS are very low. As far as we understood, you are expecting a full time PIN unlock behavior (like we have in iOS) where Enpass will never ask you for Master password even if killed by OS. Currently we don't have full time PIN unlock feature, but we do have Full time Fingerprint feature and that is for Android M devices with fingerprint sensor as mentioned here https://www.enpass.io/blog/Enpass-5-for-Android-Fingerprint-support-Autofill-and-much-more/.
Shortfall of Full time PIN unlock in Enpass is for the sake of security of Master password because we don't have any Android-provided approach to secure Master password on pre Android-M devices. More at: https://www.enpass.io/unlock-using-fingerprint-in-android-marshmallow-security/
We can understand that at times, it becomes cumbersome to provide Master password but currently we don't have any secure way to implement this feature. Yes, other password managers (mainly Online one) can do this by authenticating your unlocking attempt by saved tokens, but being an offline password manager, Enpass has no such option (currently).
Cheers!