It appears that a stack of units uses the lowest sight radius of the stack, which would be a bit unusual if that's how it's intended to be.

Here's my sovereign:

Here he is with a basic guard stacked with him:

It's somewhat subtle in those shots, but it's much more noticable in-game.  I noticed this in an earlier game tonight and started another game specifically to test it.  It's appears to be easily to reproduce, just stack a basic guard on your sovereign.  It seems someone would have noticed this by now, but I searched the forums and browsed kryo's bug list and don't see it.