I learned the trick of how to shuffle albums in a smart playlist: Remove Duplicates of [Album] - Shuffle - Expand to Full Album
I think I know the answer to my question but I'll explain the (what struck me as) odd behavior first.
For multiple disc albums I want to shuffle each individual disc, not the whole set of discs. However, I give the whole set the same [Album] name and let "disc 1 of 5" differentiate the discs - I don't like putting added text like " (Disc 1)" in [Album] field - it's the whole set that has the single name.
I thought I could be clever and put " (Disc 1)" in [Album Sort] and do: Remove Duplicates of [Album Sort] - Shuffle - Expand to Full Album
This works as expected for Remove Duplicates of [Album Sort] - Shuffle but with Expand to Full Album it shoves all the discs back together into a single set. The Shuffle on the single [Album Sort] tracks is not maintained.
I think the answer is because Expand to Full Album reverts back to using the single [Album] title. I guess, it doesn't know I mean "Expand to Full Album Disc".
To get want I want it looks like I'll have to add a " (Disc x)" modifier to the [Album] title for each individual disc. I see this convention used often and always disliked it. Do you title your multi disc sets this way?