Is the engine running or not? ie is the engine burning fuel and turning the prop at a low, non thrust producing RPM or is the engine not burning fuel but being driven round by the wind flowing through the prop?
In the latter case then the energy to rotate the engine is coming from somewhere; specifically some of the potential energy released by the aircraft's descent is being used to drive the engine round. This will show up as steeper glide angle ie a worse L/D ratio; the extra drag because the energy has to come from somewhere!
I agree with Checkboard; engine driving a non running engine produces more drag then a stationary prop.
Happy landings