Mesh deforms incorrectly

Hello all,

Im working on a game and I created a character for it but I encountered a problem, when I was trying to animate it.

It’s an anthropomorphical character because… humans are hard to do - but never mind.

So when animating I noticed that the inner jaw doesn’t correctly deform when I rotate the head bone, I’ve then tested only the head with a basic rig - and still the same thing happens.
I’ve recorded a video, which shows what I mean:

As seen in the video, the inner jaw is also (correctly?) weight painted.

I have already searched the web, but all I found were fixable by removing a duplicate armature modifier - but I onl have one.

I hope that anyone out here could help me fixing this - I have tried everyhing that came to my mind, but unsuccessful.

[Edit]
I forgot to include the blend file:

Hey.

It looks like the vertex group overlap values.
Yep. I’ve inspected your Blend file and It turns out that Vertex Group “Neck” still has some values in those areas.

Make sure you use “Auto Normalize” when using Weight Paint otherwise, you’ll be painting indefinitely.

Help me understand. What does Auto Normalize do, and how does it work?