I sent the following report to Apple. If you experience the same, do send your report as well.
On iOS 8, iPhone 6, the screen locks after standard time interval when one is reading a book while using a Bluetooth device.
This is true for Alex as well as standard iOS voices on Bluetooth speaker as well as Bluetooth headset.
This occurs always when the book is read after the Bluetooth device has been connected. Sometimes, it does not occur, though I cannot identify when this happens.