I take it the shadows cast on the ground are handled by the planes xml both in ALS and Rembrant modes?
They're actually really different.
ALS does not draw any 'real' shadow of an airplane (or any other scene element) - every shadow you see in ALS is an illusion of some sort. Rembrandt has a genuine camera pass in which objects are rendered from the perspective of the light source, creating a so-called shadow map, which is then used in a second pass rendered from the position of the eye to determine what is in shadow and what is not - Rembrandt does pretty much what nature does, it follows rays of light to determine shadows (except many objects are exempt from the shadow camera pass so they don't cast shadows in Rembrandt - clouds and trees for instance). Which is to say, all shadows in Rembrandt are guaranteed to be on the ground and properly projected.
The 'shadow' of some airplanes in ALS is a fake, it's a second model attached to the airplane and animated in such a way as to mimic a shadow - technically for ALS it is treated and rendered as part of the airplane, ALS has no way of knowing that it's supposed to be a shadow or where it should be, it would not disappear in overcast skies and... well... it can detach from the ground if the animation isn't done properly.
This is an airplane-model side problem.