Okay Kevin while that makes sense, obviously, I believe there is a problem with the use of the term "stack" in the rules. There is a general meaning of stack as simply all blocks belonging to one player, as in 14.1.2:
Each Player distributes a number of Step Losses equal to the affected Shire’s Value (SV) to their stack of Blocks in the Shire.
This is presumably the meaning that is applicable in the retreat section.
However, in numerous places in the Movement and Combat chapters, a stack is something different:
16.1: Hereafter, “Stack”/”stack” will always refer to one or more Blocks moving together.
16.6: To attempt an Evasion, the Enemy Player must declare
the attempt and form an Evasion Stack with a Leader and additional
Blocks, just like a Moving Stack in the Movement rules (16.1).
20.3.1: When different stacks controlled by the same Player are involved in a combat with Enemy Blocks, the stack that determined the Engagement order is the Starting Force
So when the retreat rules talk about a stack having to retreat to the same shire, it is not really clear what this stack is, not to me anyway.