I submitted a ticket on using peer reviews with rubrics, and Support says they can't replicate the behavior I reported, and yet for me it seems completely consistent in my testing. I'm wondering if anyone else can confirm, contradict, or comment on the following:
When a peer review has a rubric attached, if the reviewer provides any input into the rubric, even a single rating for a single criterion, the Peer Review is marked as completed in both the Student and the Instructor interface.
According a message you see in Canvas when setting up a peer review, "Student peer reviews will be considered complete when students have commented at least once on the page and filled out the rubric form for the assignment."
It's not the most major bug (if it is a bug), but it's frustrating that it seems consistent to me and support sees different behavior.