When a hostile mob stands on a slab during the day, instead of being sunburned, it appears to be in water and does not die. The same happens with the boats when they are on slabs, they begin to move as if there was a current of water. placing slabs on top of a mob farm reduces spawn of mobs within the farm by 20%
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We're actually already tracking this issue at MCPE-32822, so I will resolve and link this ticket as a duplicate.
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