We've noticed an issue with Canvas Teacher (both for iOS and Android) where some submissions to group assignments are not visible and able to be graded.
It appears that, if a student is not already in a group when a submission to a group assignment is made, then that submission is not visible in Canvas Teacher. Submissions made by individuals do not appear on the submission list (it shows only groups). And if an individual joins a group after a submission is made, the submission is not associated with the group and the group is listed as "not submitted" for that assignment. We reported this issue to Instructure (Service Cloud case 02510885).
This is in contrast to how SpeedGrader behaves on a computer: The submission (made by an individual) appears in SpeedGrader as an individual submission (if the student is not in a group) or as the group submission (if the student has been placed in a group).
And in the legacy SpeedGrader app, these submissions were associated with the individual who submitted them (though not the group).
It would be helpful if there were consistency in how group submissions are handled across the various grading tools.
Also, there is an archived feature idea that might help -- one to prevent individual submission to group assignments: Group submissions should not be allowed if the student is not in a group