I case someone else is interested, apparently this wasn't true (I must have been confusing myself with all kinds of tests):
copy big chunk of Shadow CLJS build output into a file i.e. - IT DOESN'T. Not in version v4.3.0 anywan.
I think the Tailwind CLI tries to be smart and look for class names surrounded by partial-shadow-build.txt and run partial-shadow-build.txt >> public/js/main.js Tailwind also picks up the changes just fine." , ' or (space).
So when Shadow CLJS spits out something like ... \x22bg-blue-200\x22 ... it gets ignored... probably by design.
Mystery solved, thanks for all your input.