Hi, I made some tests and I found out that if I walk on slabs, stairs (or any other blocks I can step up) I collide with their bounding blocks what causes issues for example with movement checking on some AntiCheat protected servers. I think it's an issue because slabs, stairs are solid blocks and we shouldn't pass them through in normal way.

I captured movement packets and saved coordinates between position when player was on ground and when player was on slab. So you can see whole process how movement on slabs/stairs works.

X: 129.98110656738282   Y: 71.0 //on ground
X: 130.01490478515626   Y: 71.0 //next block (slab)
X: 130.05979614257814   Y: 71.299995
X: 130.0843017578125   Y: 71.42
X: 130.097607421875   Y: 71.467995
X: 130.10889892578126   Y: 71.494896
X: 130.11370544433595   Y: 71.5 //on slab

I also created a simple image to explain it better (I haven't captured Z coordinate because it wasn't changing)

if you look at coordinates you can see that MCPE obviously cuts the corners.
As I said this issue predominantly affects servers with Anticheat because it checks if player doesn't collides with blocks and make some proper to check if player moves on stairs or slabs and doesn't just try to get through blocks is a bit hard and complicated.

The size of collision also depends on players speed (this test was performed with sneak) so if I for example sprint or have some speed effects the collision is a lot bigger and player for example collides with blocks behind stairs. And maybe its almost impossible to handle in server correctly to make that it won't affect legit players and together will block some cheats.