I think his mind was already being changed by the circumstances. After pressuring Zelinsky to be more compliant, Trump was pushing a generous compromise deal to Russia, but Putin has shown himself unwilling to compromise.
If Trump actually wants to be bring peace to the war, he needs to be tough on both sides. He’s tried being tough on Ukraine, but that wasn’t enough to bring peace. Now he wants to try being tough on Russia to force a peace. It’s just the other side of the same strategy.
It is possible that Putin thought the US would just let him take the whole country, so maybe he’ll be more willing to negotiate in good faith if he thinks the war will be much harder for him after all.
From Putin's perspective, he rules Russia with an iron fist.
So if he withdraws Russian troops from Ukraine, it is a signal showing his people that he is weak.
Putin knows he cannot allow that to happen.
Putin will continue the war until he conquers Ukraine.
Or continuing to fight the war of attrition. Even if it takes 10 years. Because that will keep him in power.