(This bug only affects worlds with the Caves & Cliffs experimental toggle turned ON). Since quite a number of versions, high grass and small dripleaves generating in lush caves only consist of their bottom half, while the top is completely missing. After some time the bottom halfs destroy themselfs because the game recognizes them as false blocks.
(It's better to see, when you set the randomtickspeed to zero.)
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We're tracking this issue in MCPE-125799, so this ticket is being resolved and linked as a duplicate.
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