I've noticed this bug for a little while now, this is a reason witches can be slightly easier to kill. They appear to be taking damage from their own poison, but they are really having the same health the whole time. Because of this, the witch continuously uses health potions to attempt to heal itself, even though it takes no damage. This is easy to show with a damage indicators mod on, but I don't have one installed. So, I've set up a slightly elaborate test to prove this otherwise.

Here's what I did:

1.) Put the sharpness 2 enchantment on a stone sword with an anvil(This makes it deal 8.5 damage total. A witch has 26 HP, so three hits will deal 25.5 damage, leaving 0.5 HP left, which is a one-hit kill)
2.) Spawn a witch, go into survival, let the witch poison you & itself
3.) Go back into creative, (This must be timed VERY good) in between the times the witch gets hurt by the poison, hit it with the sharp 2 stone sword 3 times
4.) After that, quickly hit the witch with an empty hand, and it should go down

NOTE: This can be slightly tedious as you must do it before the witch can drink its health potion in the middle of you hitting it. Also make sure to let the poison deal at least 1 damage to the witch BEFORE hitting it, and hit it AFTER every time it gets damaged from the poison to avoid the poison being dominant over the sword's damage.

Now this test shows the witch does NOT get hurt by its own poison because if the witch took a single point of damage from the poison before hitting it, it would have died by the time it got hit 3 times with the sword. The last step isn't totally necessary, just to make sure the witch is at 1 health.
Of course this test would be MUCH easier to tell with if one used a damage indicators mod, but I still need to figure out how to do download it, so I had to improvise. By all means, use it if you have it. I know this is very elaborate, but I couldn't find a better way to show this that worked as well.

-I also remember in the first snapshots the witch was introduced in, it would appear to take poison damage, and NOT heal itself. If this can somehow be reverted, then it can be fixed.

Edit - Renamed the issue due to a comment saying otherwise: Witches actually take about .15 damage from their own poison, due to being 80% immune from poison effects. This still poses the same issue, as the witch should not be healing itself if it has not lost one heart of health. It poses another issue also: Witches should not be taking damage from their own poison at all.