This is a bug or rather a design flaw that has been in the game forever, the basic is that if you place anything over a slab that is in the bottom position blocks will float over them you can see the screenshots bellow for a visual of what this looks like, to achieve this flaw you need to first place slab in the bottom position meaning that when you stand on the slab you will be exactly .5 unit from the next Y unit for example if you are on a slab that is placed on a Y level of 4 and you stand on it your Y level will be 4.5 as that is .5 unit from the Y level 5.

The only block that are not affected are slabs (both slabs must be the same variation)

Expectations are that the blocks would be on top of the slab even when it is in the bottom position this flaw does not occur on other type of blocks such a torch, button, lantern and many more as those can't be placed on top of bottom slabs but would be expected to work as well example rails on top of a bottom slab and then you would still be able to ride on them, i know this is a old flaw that has been in the game forever and some might think that this works as it should or is not worth to spend the time fixing it but i think otherwise, at the end of the day they made block waterloggable so i see no reason why they could not make block slabbable thats it for me. jeb pls fix