Verticles running into random direction when trying to create a shape key on rigged mesh

Modifiers on the mesh I’m trying to add shape keys to:

And my issue is that, whenever I’m trying to reposition multiple verticles after putting the mesh in a pose I want, they begin sliding in semingly random directions.

Any idea why and what would I need to do, to stop it ?

I first thought that you have not activated the display in edit mode, but you have already thought about it,

What is not correct is that you move the points along the normal, try global


Solved arleady. Apparently it was all fault of mirror modifier, just had to first create shape key, then add mirror modifier