In the legacy assignments tool, students could post comments at any time, regardless of the grade posting policy of the associated gradebook item. Now it appears that students can only post comments when the grade posting policy is set to automatic. I submitted a ticket for this issue and was advised to submit a feature request (which suggests that Instructure does not regard this as a bug). The support engineer also provided this workaround--students can post comments by navigating to the assignment via the Grades tool instead of directly from assignments. This is counter-intuitive. If students can't post in comments in the Assigmments tool, why would they think to try posting by going to the Grades tool. Also, the only reason this works is because the Grades tool takes the student to the legacy version of assignments. Presumably that will change at some point.
I can't think of a good reason to prevent student from commenting when the grade posting policy is set to manual. The legacy tool prevents students from viewing instructor comments until grades are posted and that makes good sense. The restriction on student commenting does not make sense. If this is not a bug, can someone from Instructure please explain why students are not permitted to post comments when the grade posting policy is set to manual.