There's a bug that when you get slower while flying with an elytra, you take fall damage or you instantly die (I don't really know). In my example, I got slower until I traveled 5 blocks/second, but I still died with full hearts. I hope you'll fix this issue.
MC-196787
Elytra Bug
Resolved
Awaiting Response
0
Feather Falling won't protect you from slamming into solid objects. It only reduces damage from falling. Is it possible that's what happened?