When a gliding player (with elytra) is relatively teleported mid-flight the elytra flight is interrupted and the player falls.

Can be reproduced using a command like this (by a player):

/tp @a ~ ~ ~

When used in a command block the command has the old (uninterrupted) behavior. If the teleportation is called by a player or if the command is within an execute command in a command block it interupts the flight.

/execute @a ~ ~ ~ /tp @a ~ ~ ~