This is likely due to the fact that the elytra still allows the player to fly underwater, with firework rockets.
The only valid configurations seem to look like this:
However, setting the graphics preset to ‘Fabulous!’ fixes the issue:
Still occurs in 26.1 snapshot 2:
The video in the issue mentioned has a similar visual flicker of blocks when pushed by a piston
Would MCPE-206165 describe your issue?
Please create a new report here, as this is the Java Edition tracker.
This includes items held in the player’s hand too:
Potentially relates to or duplicates MCPE-233012.
This appears to either duplicate or relate to MC-262617.