If there is no sorting, then the order of the cites in the citation should be taken as the intended order given by the author.
There really isn’t ambiguity in the Spec here. On this point at least, the spec is clear.
Grouped cites maintain their relative order, and are moved to the original location of the first cite of the group.
When grouped cites are moved, works by the same author are gathered together. It can change the order. It’s supposed to.
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). The moving only happens within a single citation group. So if I do
[Prefix][A, B,C][Suffix]
I may end up with [Prefix][A,C,B][Suffix] if cites are grouped.
But if I do
[Prefix][A,B][, ] [C][Suffix] (i.e. I separate into two groups)
I won’t get that.
A style that provides for sorting, or grouping, or collapsing cannot guarantee that cites will appear in the same order they are entered by the user within a single citation-group, but there is always an easy way for the user to set them straight.
The alternative is worse, because it requires the user to know what order the cites should be in and if they are wrong to fiddle with the text.
If any change were required here to meet the logic of your concern, it wouldn’t be to demand sorting as a precondition to grouping, it would be to say that BOTH sorting AND grouping should be suspended iff there are non-empty prefixes or suffixes to the citation group (just as collapsing is suspended if there is a locator). That might make sense, but my hunch is it would as often do a bad job of reading the author’s mind as a good one.