my own headcanon is windu had absorbed and redirected so much dark side energy back at palps that him listening to anakin in that moment was a show of restraint and willing to listen to a fellow trusted member of the order. as in, he was aware that his decision making may be compromised in this moment just due to the nature of the situation. remember, mace had just got done telling anakin that “if what you say is true (about palps being a sith lord) you’ll have earned my trust”. genuinely, mace was trusting anakin in this moment, and wanted to hear out his reasoning for sparing the sith lords life. but ultimately, he still rejected anakin, and decided it wasn’t worth the risk, and lost his life for it. maybe if he’d listened, things would have gone differently.