If the suitor is not reassured after the Istikhara prayer, does this mean there is a flaw in the prospective fiancée, or that God has not destined them to marry for another reason?

The believer's entire affair is good. If something good befalls him, he is grateful, and if something bad befalls him, he is patient. What was meant for you would not have missed you. Perhaps what happened contains much good, as Allah Almighty says: "And perhaps you dislike a thing and it is good for you." And the breaking of an engagement before marriage is easier than breaking it after. Therefore, you must be content with Allah's decree and destiny, according to the hadith: "Whoever is pleased, he shall have pleasure, and whoever is displeased, he shall have displeasure." Turn to Allah to remove what troubles you and to choose what is best for you. Occupy your time with beneficial and useful things, and distance yourself from negative thoughts.