I don´t know if it is because I use RUb 1.43, but in my test mission they engage at 6000m. Perfect weather, 1.Jun.1941, 12:00, no wind. There´s the Bismarck (docked) and the King George. It´s 8000m away and drives towards the Bismarck. The Bismarck almost immediately detects it, but waits until it´s 6000m away...
Please try this mission and let us know the result.
http://home.arcor.de/gizzmoe/files/Bismarck_Test.rar
Also please send me your mission, the address is gizzmoe [at] gmx.de.