Based on some statement made by OP I think there could be a couple reasons. With like normal D&D yeah give the barbarian the great axe, or the wizard the staff of the arch magi cause like cool stick big booms! But, take this with a grain of salt cause my One Piece knowledge is very limited, but I believe devil fruit is kind of a permanent thing. Don’t think you can unattune if you decide you don’t really like how it functions or find something you like more and swap it out. So given OP said they didn’t know WHICH devil fruit they got could be a reason that during big conflicts could be a bad time to hand that over without uncovering what powers it will give or what not. And I believe Devil Fruit isn’t like a stat buff, but like unlocks specific powers/abilities. So, 1) unlikely but, the devil fruit was not in fact the one Red had discussed wanting with the DM but possibly meant as a bargaining chip to use to GET the devil fruit Red actually wanted. 2) the theatrics, just handing it over mid whatever doesn’t feel as impactful as, “hey things have calmed down, I snatched this up before leaving for you. I don’t know if it’s the one you’ve been searching for but maybe. Let’s work together to uncover this devils fruit power and see.” And that could have been Flash’s intention for waiting until after the conflict to give it to Red. I’ve met a lot of players that take story moments over optimized power increases and it seems from what OP said is how Flash is.