MCPE-54855 dose not describe the problem, it has nothing to do with the placing in a line feature that bedrock has.
I was just trying to fill a top half slab to a full block by clicking on the bottom of said slab, but if there is a top slab of the same type above the block I'm trying place one, it's like the the slab I'm trying to fill doesn't exist and just places it on the slab above.
I first saw it on bedrock realms, I then tested it on a singleplayer map to the same result. I'm playing on window 10 version on PC.
I could also add a video of what's happening if that would help.