Drinking any Minecraft potion will give me the thirst effect.
Please fix this.

Issue still isn't fixed in 3.1.0 - 129 unfortunately

A hacky fix you could do is to go to the file: <modpack_root>/config/toughasnails/drink_stats.json and add the following to the JSON array:

  {
    "item": {
      "name": "minecraft:potion",
      "metadata": 0
    },
    "thirst": 4,
    "hydration": 0.3,
    "poisonChance": 0.0
  }

You might need a comma in there....

...

Actually, that might not work.. I think ToughAsNails will intercept the potion usage before that data can be applied...

Also.. after looking at the code a bit more it doesn't look like that can happen at all unless something with Forge changed. minecraft:potion items that have NO effects should give the player thirst, but potions that have effects shouldn't.