The Bug:

When walking over a block that has a falling block entity inside it, and the falling block has the same blockstate id as the block, it causes very slow and glitchy player movement. Only seems to affect non full blocks like slabs.

Expected Behavior:

Normal movement over the block just like any other solid block.

How to Reproduce:

• Put the following command in a Repeating Command Block that's set to "Always Active":

  execute as @e[type=minecraft:falling_block] run data merge entity @s {Time:1}

• Run the following commands: 

  /setblock ~ ~2 ~ minecraft:oak_slab

  /execute align xyz run summon minecraft:falling_block ~0.5 ~2 ~0.5 {BlockState:{Name:"minecraft:stone_slab"},NoGravity:1b,Time:1,DropItem:0b,Invulnerable:1b,PersistenceRequired:1b}

  →  Walking on the oak slab causes no problems because the falling block has a different block id than the oak slab.

• Now move to a different area and run the following commands:

  /setblock ~ ~2 ~ minecraft:oak_slab

  /execute align xyz run summon minecraft:falling_block ~0.5 ~2 ~0.5 {BlockState:{Name:"minecraft:oak_slab"},NoGravity:1b,Time:1,DropItem:0b,Invulnerable:1b,PersistenceRequired:1b}

  →  Walking on the oak slab causes problems because the falling block has the same block id as the oak slab.