I spotted this bug in my game, though it wasn't consistent,

I cast shrink on an escaped juggernaut and his attack became zero.  When it was his turn he just stood there for a moment and then passed his turn.  That has been consistent between saves for me.

Had the same problem with almost all of my heroes shrinking in size when I used the growth spell. Couldn't note down why some would actually grow, while others would shrink with this spell.

Actually for me it was my sov casting shrink on the juggernaut.  Don't have the mace.  Display showed an attack of zero.  Growth seemed to work fine on Othollo in the same battle.

I think the growth spell shrinking units might be to do with not having larger animations of those mount types.

I have to agree, based on my experience with 3d games the size or scale of the unit should have nothing to with animations. If casting on self and casting on other are treated in opposite ways, then maybe its something more substantial than a typo in the xml.