Any knot is going to be a weak point compared to the rest of an undamaged and quality manufactured rope. But a damaged/poorly manufactured rope may have weak points (possibly not being obvious to an observer) that are worse than the knot weak point. The bends and twists (the stress points) that are inherent to a knot are never going to make it stronger than the rope it is tied with. It will always be weaker. Your rope that broke away from the knot may have been damaged by the previous knot at that location (that was later untied). Or it could have had internal flaws that were not noticeable.