I"m not sure whether it is a bug or not, but I will try to describe the situation. I have a card which I turned it to a component with 4 properties i.e. Link, Heading, Description and Icon visibility.
Before creating the component, the card had two global classes.
Card-thumbnail: creates the structure, background image and etc.
Card-hover: Creates a simple hover transition.
Now after turning the card into a component, everything works well. Except when I assign a custom link property to any of the instance. Immidietly, it looses the background image. Moreover, when I unlink the instance to add a custom background image with a combo class, it also looses the hover interaction from the global class .card-hover.
I had to delete the hover class to set it again outside the component. Seems like it is disregarding everything that was set inside the component.
Also the CSS preview does not work on components 