@theluthier Can you take a look into my current Piero file? I can see the same problem with your Piero already around 9:48 in this lesson video and later at 9:52 as it is visible in this screenshot:

Do I see it right that I can manually change the location of every vertex on every position of the shapekey slider not only at the default maximum influence value of 1 for every relative shapekey?
The eyelid shapekeys are really tricky. No wonder that three quarters of this lesson deal with them.