(I think Paul is right that the spec text is currently contradicting the test unambiguously, so this is about how CSL should ideally work, not what the current spec says)
It’s really not a problem for the user, any more than sorting (which also disturbs the order of citations compared to the order in which they have been entered).
I think that’s the crucial point here, though. Sorting can absolutely be a major issue. Zotero (at least) has an option to disable sorting for styles that do normally sort, and having such an option is, imo, crucial for when citations are ordered by some sort of logic. This is often associated with affixes such as in @bwiernik’s example but need not be.
Most author-date styles do sort, Chicago (author-date), the style in the test, does not sort. The reason is that the style guide specifically states that order of works in a citation may be meaningful (e.g. most important work first, etc.). So there absolutely needs to be a way for authors using Chicago style to maintain the original order of citations, including in citations that might otherwise collapse.
The current test behavior is maximally flexible, if possibly tedious for an author: if the author wants the citations collapsed, they can move the item within the cite. Unless I’m missing something (and that’s possible), absent further modifications or flags, your implementation makes it impossible to get (Aalto 2015a,b; Bartleby 2010; Aalto 2015c–e) in Chicago style. I think that’s a bad idea.