Not sure why, but focal length keying worked for me in Blender Version 2.79 (2.79 2017-09-11, Blender Foundation), but does not work in Version 2.79b (2.79b 2018-03-22, Blender Foundation).
Sep 25 2018
Sep 9 2018
Jul 18 2018
Thanks for trying to reproduce it. As I noted in my second comment I did test on 3.9.1. Strange that its working for you, let me investigate more.
Jul 16 2018
I've updated the steps for reproducing the bug to set simplify to 0.
Jul 15 2018
Confirmed the issue is present in 3.9.1 of the FBX exporter.
Jun 4 2018
Jan 17 2018
If we're worried about collisions, maybe we could replace the ID with some sort of UUID or content hash when it exceeds the limit. I'm not really sure what the side effects of changing the ID are, but the import seemed to work fine as far as my needs go.