Speedgrader should display ungrouped discussions, or not allow students to post to a group discussion until they are assigned to a group. The present state allows a third configuration that is broken.
If an instructor creates a group discussion thread, and a student posts in the thread before they are assigned to a group, their post is not (and will never be) visible in the speedgrader. Instead, the speedgrader unhelpfully displays this message:
"The submissions for this assignment are posts in the assignment's discussion. Below are the discussion posts for [studentname], or you can view the full discussion."
This is inconsistent with group assignment submissions, which will successfully display a submission regardless of whether it was submitted by an individual student, or on behalf of a group.
Canvas should either make the discussion grading experience consistent with assignment submissions, so that ungrouped posts appear in the speedgrader, or block students from posting in a group discussion if they are not assigned to a group.
Allowing this third outcome creates frustration and wasted effort for instructors who will now have to alternate between searching for student posts, and entering them in the gradebook, instead of being able to easily find a student's posts with the speedgrader.
I appreciate that canvas allows a high level of flexibility, but I wish they would put some guardrails in place to protect instructors from configuring assignments in ways that do not work. If canvas does not intend for students to comment on a group discussion before they are assigned to a group, the platform should not allow that to happen.