I just came across a difference in sort order between a Group in the sidebar and a sorted custom report. In the Group sidebar they were sorted by last name, given with individuals with no middle given name appearing before those with a middle given name.
In a custom report based upon the group and sorted by surname and then given name, the individuals with middle given names sorted before those without.
Either there are two sort functions being used or a collation sequence is different between the two sorts.
https://postimg.cc/gallery/2tnmls6qk/