In reply to Tudor Timi:
I see. So this is called a “Vacuous Pass”. When there is no match in the antecedent, then evaluation of the implication succeeds vacuously and returns true.
I also found out that in order to avoid this, a cover property is used since it does not cover vacuous passes.
Thanks for the help. I finally understand this issue and why a cover property is sometimes used together with assert property.