When on a 3x3 plane without the block in the middle you place vines in the empty block, it adds to the block walls depending on how many vines you placed until it reaches all the walls. When breaking them with the shears, it turns into as a single block wich only drops 1 vine and eats the other 3.
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We're tracking this issue in MC-123140, so this ticket is being resolved and linked as a duplicate.
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Duplicate of MC-123140.