The inspector UI for the expression parameters does not mark the asset as dirty, so clearing or defaulting through the UI doesn't get updated to disk when saving the project.
Previewing and uploading the avatar worked, but only during the same session of the Unity Editor.
As a result, this broke my Two-Axis Puppet relying on VRCFaceBlendH/VRCFaceBlendV, as I'd previously created an empty expression parameters object then defaulted it, to work around the default VRCExpressionParameters ("DefaultExpressionParameters") object erroneously consuming 128 bits of expression space (