Did you try, first, to uninstall any mods you might be using, and try again? If you don't have any mods running, and this is still happening, I have an idea what might be causing this. I'll have to investigate.

EDIT: Yep, it is as I suspected. Due to a bug of some sortwith the previous implementation of 'stack crafting,' they wanted to 'fix it' by changing the ingredients required for crafting the Thunderbolt potion. In doing so, however, they made the ingredients listing identical to the one used to craft Necrophage Oil. In TW2, you can't have two different crafted items share the exact same ingredients list; the result is that only one of the items will be crafted - in this case, you get a Thunderbolt potion instead of a Necrophage Oil, because the alchemical components needed are identical for both.

What they did was to change the Thunderbolt recipe - previously 1 hydragenum, 2 quebirth, and 1 sol - to replace 1 quebirth with 1 vitriol. This made it a duplicate of the Necrophage Oil recipe, and that's why you're getting the potion instead of the oil when crafting.

It's a pretty silly oversight on CDPR's part, since this is something the mod community has been aware of since the beginning, and something they always have to keep a lookout for when changing crafting recipes. I posted a report on this supposed bug on the official forums, as well.