To make a slowness IV potion (added in 1.13) You have to first corrupt normal Speed potion then add a level modifier. corrupting speed II doesn't work as it does for other potions.
MC-159341
You cannot corrupt speed II potions
Resolved
Invalid
0
Thank you for your report!
However, this issue is Invalid.
You have posted a feature request or a suggestion. This site is for bug reports only.
For suggestions, please visit the Minecraft feedback site or Minecraft Suggestions on Reddit.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki