A mob with elytra in the chest slot, with the FallFlying:1b tag, does not glide when it unmounts from an entity in mid air

try this command:

/summon zombie ~ ~100 ~ {Passengers:[{id:"minecraft:creeper",FallFlying:1b,ArmorItems:[{},{},{id:"minecraft:elytra",Count:1b},{}]}],ActiveEffects:[{Id:6b,Amplifier:5b,Duration:20}]}

it will summon a zombie, with a creeper riding it, the zombie will die instantly and the creeper will unmount, but will not activate the elytra when falling.

that's all I think