Strong potion of slowness gives slowness level V

I do not think this is a bug. With the addition of the Turtle Master Potion (Resistance IV and Slowness IV), Level V Slowness is just enough to remove the Slowness IV from the Turtle Master Potion and leaving the Resistance IV effect for a considerable amount of time. Combined with the fact that slowness II potions never existed in the game, I do not believe this was just an accident or a "bug". With this in the game, it rewards the most knowledgeable players with something very interesting. Minecraft needs more game mechanics like this.

It's not at all balanced.

No it is not balanced but the concept is still very good.

Still in 18w19b

Changed to slowness IV instead to balance it. The short duration is also meant to balance it. 

Then you fixed it giving slowness V. 🙂