Title says it all.

With this mod installed, the Purified Water Canteen has a chance to give you the unwanted Thirst effect.

Tested multiple times without the mod, purified water doesn't give you thirst (duh).

With WaterSkins, it has a chance to give you thirst as if it wasn't purified.

Hi,
As this is a cosmetic-only mod, it does not affect or override any functionality that Tough as Nails provides.
How were you able to reproduce this?
If you look closely at the code you can see that it's only a simple value calculation based on item ID and some display code.

It's easy to reproduce.

1), without Waterskin installed, use /give [playername] toughasnails:canteen 1 2. This will give you a full purified canteen.
2) To make yourself thirsty: /effect [playername] toughasnails:thirst 3 100. Now, drink from multiple purified canteens or other purified sources. You'll notice that you never receive the "Thirst" effects. Because it's purified.

NOW,

3. Install Waterskin. Grab yourself a Purified canteen, and notice that there is now approx a 1/3 chance that when you drink from it, you will get the Thirst effect, as if it's not purified water. This is only occurring with Waterskin installed alongside the Tough As Nails (TAN) mod.

What should be happening: Purified water shouldn't, and hasn't, caused the unwanted "Thirst" effect when drinking from purified water sources. But for some reason, there's a chance (the same chance as with regular unfiltered water) that with Waterskin enabled, you will likely get the Thirst effect for some reason.

I just tested it again and can confirm this is still the case, even with the only two mods installed being TAN and WaterSkin.