On mobile, when you select/hover over a block, the game remembers that offset position from the player and keeps selecting the block at that offset once you move. It resets after selecting a new block or using the hotbar. It can be used to select blocks and entities through walls too, letting you ride, trade with and tame mobs through walls.
We're tracking this issue at MCPE-140671, so I will resolve and link this report as a duplicate.
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