I recreated my controller and reran the same test with juju 2.8 and I didn't see the issue at all. Just in case there had been some issue with my first controller I recreated the controller back at 2.9-rc3 and reran again and the issue appeared again:
Run 1: 0 failures
Run 2: 0 failures
Run 3: 1 failure
Run 4: 5 failures
Run 5: 3 failures
I recreated my controller and reran the same test with juju 2.8 and I didn't see the issue at all. Just in case there had been some issue with my first controller I recreated the controller back at 2.9-rc3 and reran again and the issue appeared again:
Run 1: 0 failures
Run 2: 0 failures
Run 3: 1 failure
Run 4: 5 failures
Run 5: 3 failures