One thing is for certain God did not lie, b/c God cannot lie (Num 23:19; 1 Sam 15:28-29). It goes against His very nature. Now in regards to your question—where do you get that God decreed that Hezekiah would die? There is a difference between a divinely decreed statement and a statement of divine intent. Decrees/sworn declarations are unconditional & bind the speaker to the stated course of action regardless of the circumstances or reaction of the listener (cf Gen 22:16-18; Ps 110:4). Ie they come to pass regardless of any other circumstance. A divine intention on the other hand retains a conditional element & does not bind the speaker to a stated coarse of action (cf Jer 18:8-10; 26:13,19; Jonah 3:9-10). Ie the outcome can be altered based on the reaction of the one to whom the statement is made. I personally put 2 Kings 20 in the divine intent category. But regardless of this linguistic difference between us, I still don’t see how it could be assumed that God did not know what Hezekiah would do. If we take into account Ps 139:16—we see God knowing all that will happen to us even before it actually happens. Job 14:5 tells that God even knows the number of our days (or lifespan). Its not as if God did not know Hezekiah would pray & would not die at that exact time (this would contradict both Ps 139:16 & Job 14:5 if He did not know). But instead, the real question becomes how did Hezekiah’s prayer legitimately impacted the situation —or how does prayer work in relation to an all knowing God—who can know our needs even before we ask (Matt 6:8), yet not met our needs until we ask for His provision (Jam 4:2). But back to the question at hand, IMO either God is truly omniscient & knows all (even that Hez would pray & not get 2 Kgs 20:2 immediately fulfilled) or He can be taken by surprise & have His eternal omniscience thwarted by mans actions.
Should there have been an asterisk next to that decree?