It's clearly a bug, but what is the bug exactly?

A couple weeks ago I was updating some of our Playwright tests, when Playwright decided to mess with the execution order of the tests. The tests in question are a group of tests that cover a whole process from start to end. So we run them in serial mode , "to ensure they will always run together and in order" . And yet, they were not running in order. While that's obviously a bug, it wasn't clear…