If you preview a blend shape and then swap to none, the value isn't reset to 0. It stays at 1. This makes it easy to miss that you've changed something that you don't intend to have changed. Since it saves the preview value to the default value on the mesh renderer on save. Which is not great

One confusion moment for example.

* Set Eye lids to blendshapes since you're unsure first time

* Set up the mesh and the blendshape, click preview on blink

* Realize your mistake, swap over to Face tracking.

*Upload.

The avatar is now uploaded with eyes closed. Since the preview doesn't clear.