Yep, and it's not just Mossy Cobblestone wall. All walls/fences have this issue as well. It could be something to do with the larger hitbox.
I can confirm this. Spawned in a jungle on SSP with some pretty broken trees.
Seed: -948410306619472960
You can't miss them. You literally spawn above one.
I couldn't replicate this (I don't think) When I entered the Nether, I still had the Speed buff, but my walking speed seemed to be set to normal. Sprinting re-activated the buff's effect.