So I am in both camps described above:

First time I cancelled, points came back almost immediately, and taxes and fees were refunded in a couple of days.

Because of that, this time - three days ago - I foolishly did not bother to take a screen shot of the page stating that the refund would be calculated and be forthcoming. This time, the miles did not re-post right away (nor has anything hit the credit card). I realize that it could take another week or so, but I have no evidence that I have cancelled the flight (other than it disappearing from my Bookings) and with the flight scheduled to take off tomorrow, I find myself wanting some proof that I did indeed cancel before takeoff. Any ideas?

Update: I went onto the Flying Blue website and eventually found a 1-800 number that went, for reasons I do not understand, to France, or at least someone who was speaking French but not English (and was probably not a French native speaker, but I'm no authority). Anyhow, after a little back and forth, with me expressing my concern about not having any verification that I had in fact cancelled the flight, the person with whom I was speaking said that the problem was that because they needed to exact the 50 dollars or euros per ticket for a refund, that cancelling on line was not an option. I grant that that contradicts my other experience as well as the experience of several of you. I was ready to fork over the money to get the process moving when this nice person said that because ours was a health-motivated cancellation (Covid), they could waive the fee(s). That was nice, and I said so. A few minutes more of me answering random questions and, amazingly, the points were back in the account. I don't see a credit back yet on the credit card, but I was hardly expecting that instantaneously.

What have I learned from this experience? Doesn't hurt to try the web site cancellation route, but if that does not pay out in points immediately, should probably call in to try to move things along.