When placing blocks while crouched the aim is off causing the block to place incorrectly. If you aim at the top of a full block and try to place a slab while crouched, it will often place above the block space even if you have aimed correctly at the top half of the full block you are trying to place the slab against.
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We're tracking this issue in MCPE-167559, so this ticket is being resolved and linked as a duplicate.
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I have this issue in Bedrock Edition 1.18.7102.0 on PC. Crouching and aiming the crosshair to place blocks on an object that's not a full block will cause the block to be placed in the wrong place beside, above, or underneath it. This happens to Chains, Stairs, Slabs, and other blocks that I'm not aware of. In the video I uploaded, I tried to place a Lantern at the bottom of a Chain, and when I was clearly aiming at the bottom of the chains, the Lantern is placed beside it instead.