When you put on your Elytra __ and you go inside of Lava *and then you hold down ++***X (that's the letter on the controller and it means "Jump"), you'll fly with the Elytra that you put on.
It doesn't make any sense at all because you aren't in the air.
Thank you for your report!
We're tracking this issue in MCPE-19600, so this ticket is being resolved and linked as a duplicate.
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