Mud doesn't seem to be a full block, in terms of the Y coordinate. I noticed while working in my swamp base that if you're standing on a mud block:
You can't then walk up stairs or slabs next to that block. It's as if the stair or slab is more than a block tall (and thus can't be walked on)
When you step onto the mud block, your Y coordinate decreases by 1, as though you had fallen a block (I think that's why the slab and stairs don't work). It looks visually like a whole block, same as the dirt and stone blocks next to the mud, but acts like a slab or as though there just wasn't anything there.
Thank you for your report!
However, this issue is Working as Intended.
The report you have submitted is working as intended: when walking on mud blocks, entities sink 2 pixels into the block, exactly the same way players sink into soul sand.
Please note, that mechanics of the game may change between updates.
Things such as graphics, sounds, world creation, biomes, redstone, villagers, and animals may not work the same in current versions.
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