I couldn't recreate this bug but I walked a strider off a slab over lava and it was floating in the air. It was also displaying the "shivering" animation.
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Thank you for your report!
We're actually already tracking this issue in MC-176032, so I resolved and linked this ticket as a duplicate.
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Can confirm this. It is not just limited to walking over slabs, but trying to jump up/fall down blocks.