I use jump boost 255 to give one player on my server an immunity to fall damage. It's worked great, except now that he has an elytra, the elytra only sometimes activates, no matter how much he spams space while falling. I have been able to duplicate this issue both on my server and in a singleplayer world.
I use /effect @p jump_boost 1000000 255 true to give the jump boost.
It seems to work more at lower elevations (such as underground) and also after falling for a longer distance.