Up-down slime block elevator clones half-slabs temporary, and slime blocks deal damage while moving down

I made an elevator like in the youtube video "Minecraft 1.11 Cheap _ Simple Platform Elevator" from Alonofer20 at "https://www.youtube.com/watch?v=0WzJBkhjqrQ", but it went up glitching a bit for about 1 game tick, seeing through all blocks every time it goes up a block, but for going down, it started to appear ghost clones of the half slabs while it went down, at random Y positions and X, Z positions, and I glitched through them and die several times, I also tried to break them, but they give no block, although I can step over them, and also while going down only with the slime blocks, I started to bounce and several times I took a lot of damage and I ended up dying like if while going down the slime blocks were solid blocks, so this might be also a second bug. Also after reloading the map all the ghost blocks were gone, but after going up and down the elevator cloned the slabs again, making it useless for now.