I am using peer review for the first time in my classes this semester (it's been going really well! It saves me time grading and commenting, and I think students are learning better, from what I can see as I assign the official scores in SpeedGrader), and several students have run into this problem:
After having filled out the rubric part-way through (I have them enter scores and comments directly on the rubric), they click on "Save," thinking they are saving their progress and they can come back later to finish it. But when they try to edit it, that's when they realize "Save" has worked like "Submit" and they can no longer change the rubric.
As a work-around, I have told my students to message me when that happens. I have found (can't find the reference right now) that if I delete the assignment of peer review and re-assign (the same work to the same student), they can edit and re-submit the rubric. I see there have been similar comments on this feature in the past (Edit or Delete Rubric Comments and Allow students to edit their assignment peer review after saving it), but they are either slightly different (it should be a super simple change to change "Save" to "Submit" at minimum, right?) or the idea is in cold-storage for some reason.