Head movement manipulates face bones (or possibly visemes)

[Morfium]

Hi, I'm kinda stuck. :)

I successfully converted a character to VRChat. Everything was working okayish. Then I learned how to UV map. Since I wanted to make my own textures I updated the avatar.
Actually I completely redid it because I learned a ton of new things. I added a facial expression that works well.
But now whenever I lower my head (like looking down) my eyes close. I mean, when I tilt my head very little, the eyes just start closing. And when i tilt it further the eyes do the same. When I look down they are closed.
I had the same problem with my mouth opening on tilting my head down.
No idea how I fixed that....
I can't find any relation between the eyes (it's a viseme that cloeses the eyes) and the head tilt.
It's a MMD char and I used CATS tools to add blinking and decimate it. But my old char was made the same way and blinked just alright.
I'd appechiate any notch in the right direction! :D

[Daffa]

100% working fix! Friend showed me.
Ensure in Shapekeys(Blendshapes) it looks like the following:
vrc.blink_left
vrc.blink_right
vrc.lowerlid_left
vrc.lowerlid_right

You can move the shapekeys(circled in blue) into the correct order with the arrows(circled in red).
https://i.imgur.com/G66mL2k.jpg

If they aren't in the correct order, VRChat will make your mouth open randomly because it's using the wrong shapekeys.

[Morfium]

Oh cool, thanks for the reply. :D
I'm not even sure if I fixed this already haha.
If if I didn't I'll try this, thanks! :)

[SHOKK♥]

Summary of what causes this issue to understand and avoid it in the future.
It's looking for a Lower eyelid shapekey, if it's not there, it takes what is in it's place, usually a facial expression. Which is why some avatars open their mouths when they look down/up.
The solution is having just a empty shapekey with the names

vrc.lowerlid_left
vrc.lowerlid_right
.
So you want those 4 shapekeys at the top under basis. (even if you make your own blink animations, just have them blank like lowerlids)