So I also have another phone, an S7 (non-edge), and it's running Android 6.01. Until earlier this evening it had version 110.0.0.15.69 of the FB app installed... which did NOT demonstrate the notification sound retention issue. The device has always worked perfectly in that regard.
Since I just became a beta tester for the FB app, I went ahead and updated to the beta version on my S7, too. So my S7 now has version 133.0.0.16.83 (beta) on it like my S7 edge. And guess what? The notification retention problem now happens on it as well. So the notification sounds that have been selected are forgotten by versions 132.0.0.20.85 and 133.0.0.16.83, but not by version 110.0.0.15.69. And that more or less cinches it for me that the problem is with the newer releases of the FB app, itself.
I've expressed this to the developers. I only hope they take observations from their beta testers seriously. We'll see.