Prophecy, as I have written before, is primarily about God’s power. God says something will happen and then uses His power to make it come to pass. Prophecy fulfillment is about validating that power. Prophecy is not about knowledge.
In Jonah we find God declaring destruction against Nineveh, but never fulfilling His promise. The text says as much:
Jon 3:10 And God saw their works, that they turned from their evil way; and God repented of the evil, that he had said that he would do unto them; and he did it not.
The text spells it out. God said that He would do something. God repented. God never did it.
If prophecy was about knowledge then this would be a problem. If God was illustrating His grasp of future events, the prophecy of Nineveh is a complete failure. If God knew that the prophecy would never come true, this prophecy would also amount to a lie. God said He would do something, knowing He never would. There is no hint that the prophecy is conditional, and the people are unsure if the prophecy is conditional:
Jon 3:9 Who can tell if God will turn and repent, and turn away from his fierce anger, that we perish not?
Jonah was not preaching a conditional prophecy. Instead, Jonah preached exactly what God told him: In 40 days Nineveh would be destroyed by God. God was going to display His power. The people begged for mercy and this caused God to repent.
Prophecy is not about knowledge. It is about power. In response to God’s prophecy the entire nation turned from their evil. They truly believed God could bring to pass the prophecy. God, then, had no need to show His power. They already believed in it. If they had refused God, God would have used them as an example of His power and a warning to other nations.
Because prophecy is primarily about power, God does not mind when prophecy fails. God is not concerned about what people think of His “prediction” ability. Every time God speaks about true predictions, it is in this context. Every prophecy just assumes the future is not set, and God is actively working to bring about the prophecy. In this sense, each prophecy can be viewed as a blow against traditional omniscience. If God did know the future, His claim would take the form of “I know it will come to pass because I see the future”, not “I know it will come to pass because I will do it.” But the Bible is devoid of the former and filled with the latter.
God is primarily concerned that His power is known and His will be done. Prophecy is a means to an end. If events change, sometimes the prophecy is not needed. God then abandons His own prophecy. Prophecy is not about knowledge, but power.