This has never happened before on SE, not even earlier today in a different stack.
I wrote a question and clicked "post." Immediate pop-up "Are you sure you want to leave this page?" I assumed (incorrectly) that something had gone wrong. So I said No and tried again. oops! duplicate post.
Then I saw a typo and tried to fix it. When I clicked Save, same pop-up, with a busy spinner on the Save button. Clicked yes (leave), and saw that the edits had actually been saved.