Instructor has a moderated assignment with three other graders.

He has completed the step of moderating the assignment and accepting the grades from the other 3 graders.

He has also published the grades.

He saw the graders' comments last time he was in the SPeedgrader viewing assignments and graders' final grade and comments, but now when he goes back in, the graders' comments are gone and only his (the Moderator) comments remain. 

The only thing I found in the Canvas guides is on this page: How do I Review Moderated Assignments in Speedgrader as a Moderator?

and in particular, this note:

"...Students can only view comments in the submission that is selected for the final grade; they cannot view comments from any other provisional grade reviews. If you want to include any comments from additional reviewers, you will have to copy the comments manually into the submission selected for the final grade"

This suggests the comments from graders will not be visible to students unless the moderator copies them into the comments when altering the final grade.

If this is the case this shouldn't it be mentioned int he general information about moderated assignments but it is not 

see:How do I add a moderated assignment to be graded by multiple reviewers? 

(nothing about reviewer/\grader comments there)

The graders spent a good deal of time entering comments.

Last resort idea/question: I know you can view Gradebook history but can you restore the gradebook to pre-moderated status?