The mentioned unittest fails when the groups which it creates during its execution are named "group_9" and "group_10". Also see MDL-70110#comment-828621 where the issue was discovered.

After first testing this issue does not happen when the group numbers are 1 higher ("group_10"/"group_11") or when they are padded with zeros ("group_09"/"group_10") suggesting that this is due to some sorting.

Furthermore the tests seem to be run with a teacher as the active user. The method call leading to the error (mod_assign_external::get_submission_status) tries to get the status without passing a groupid or userid. According to the documentation the function then auto-selects the group based on user id. I assume this mechanism somehow sorts the groups and (as the teacher has access to both groups) returns the first (alphabetically).