I made a couple of Potions of Harming (Instant Damage II), normally I can add a gunpowder to this through the brewing stand for Splash versions of these.

Now I can't even place these Potions of Harming II in to the brewing stand. So am not able to add the gunpowder.

The other way around is still possible, making a Splash Potion of Harming I into a Splash Potion of Harming II by adding a glowstone dust to it.