In a transfered world. rockets are not always firing when elytra are engaged. This occurs during taking off from a one block height and higher. Flew to the end city and rockets would not propel over the void at all times. They will fire if I cursor to the ground but it appears your code while in flight or taking off with elytra fully engaged doesn't always register to be able to propel the player.
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-30821.
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