I have now had three instances, over a couple years, when I extended the due date on an assignment to give students more time to turn in the homework and then automatic peer-review assignment does not occur. This last time (link included here for Instructure folks in case they are interested: PS4 (northwestern.edu)), happened last night (11/5/21). In this instance, I extended the due date for the homework by 1 day but did not extend the due date for the peer-review, which was always set for 2 days after the original due date of the homework (i.e., for this homework, students now only have 1 day to turn in the peer-review). I assume that this is a bug as it occurs sporadically for me and want to report it here for record. While the bug is rare, it can be an annoying inconvenience when there is a short timeline that occurs over the weekend. In the current occurrence, I caught it in time and was able to go in to "manually" fix it. (I put "manually" in the quotation because the "Assign Peer Reviews" button still work; I just need to go in to the assignment and click it, which should not have been needed if the software works correctly.)
I note that I have been using Canvas and peer review regularly since 2014 now and considered myself to be reasonably proficient at setting up this task.