It's really difficult to fit into a 15 second video for GFYcat, so I guess I've got to ask that you check this one if you can:

Trodden dirt has odd speed boost issues. I think I recall that a change in 1.8 was how that was handled separately, but I don't remember for sure. Essentially, if you stand on the trodden dirt and just tap (not hold) your Forward keybind, Trodden Dirt keeps you moving forward. Additionally, the left & right movement seems boosted too, which makes staying on the dirt nearly impossible if it turns any.

Let me know if you need some confirmation or something on this, but like I said, it's pretty tough to demonstrate. That being said, this is also a tiny bug and should be on the bottom of your list of priorities IMO - I can live without it and just continue using double slabs / slabs ;)

yeah something changed in how 1.8 calls the "entity walking over block" method and I can't quite figure it out yet, thanks for bringing it to my attention though!