The issues appears when we change from a theme that contains panel shadows to one that doesnt. When the current plasma theme supports shadows and the one to change to it doesnt then the plasma panel after the plasma theme change still draws the shadows from the old theme

Steps to reproduce:

1. Install one theme that doesnt have panel shadows e.g. Unity Ambiance (does not support panel shadows)
2. Choose Unity Ambiance and re-login to plasma you will see your panels without any shadows
3. Change your plasma theme to Air
4. All panel shadows are drawn correctly
5. Change your plasma theme back to Unity Ambiance, you will notice that shadows from Air are still drawn even though they shouldnt