Android doesn't necessarily destroy your activity on Activity.Finish. It puts it in the background and when you select it from the recent apps list will bring it to the foreground. In this case Activity_Create will not run but Activity_Resume will. The fact that it is crashing means there is something wrong in your code. add '#BridgeLogger: True' to your Project Attributes and see if you get an exception shown in the logs.
To really close your app do one of the following
B4X:
Dim jo As JavaObject
jo.InitializeContext
'jo.RunMethod("finishAffinity", Null) ' one way
jo.RunMethod("finishAndRemoveTask", Null) 'another way