1- Himself  a Reincarnator: he is the biggest proof of the existence of Reincarnation why he should belive that he is the only one? like him others can abuse their knowledge of the game to obtain something and we saw this in Marie's actions to take over Livia's original Role. Even in later Arcs A reincarnator fuck up the plot because she and one of the heroins were born as twin sisters and other was born as one of the target boys their mere existence already hurt the plot heavily and their actions break it apart even before leon started to interfere. He Should be More Wary of Reincarnators
2- The World Itself: other problem  that Leon refuse to acknowledge is that the World is Real and keep saying that is "This is an otome game" it resembles the game but it doesn't mean that the situation of the world itself is because "it is an Otome Game" like he says and Luxion Warn him many time about this yet he keep dismissing this with "This is just a Otome Game" refusing to accept that his actions and Marie's actions have already destroyed the Original Plot  and this Make Leon ignore Every possible Danger until it's too Late and tha Leave him with very Few ways to deal with the problem, Unlike the Marie Route were thanks to Marie's Warning about other Sequels of the Game Leon decide to Act saving far More lifes than the Main series Route( where he Regrets the Result of the war if he only had used Luxon better all could be avoided)  but Also Aweaking Different Problems than the ones in the Main Series. This Proof that The World Answere to Leon's Actions and produce a Reaction. He should be more aware of the world reactions to his and marie's actions and act before it too late
3- his Belif that the plot will follow itself: like in this Chapter of the manga Livia points out how Leon Saw her as a pet and not as a person, the same problem lies here Leon thinks that if Livia meet with the Five targets everything will be alright because the plot says so refusing them as Persons with their own mentality and will. in Marie Route Livia Meets with the targets thanks that leon stopped Marie  what happened with livia? she Now Suffers Mental Break downs and Close herself in a dark room to protect herself from the bullying and avoid the five targets. why this happen is because Livia don't have the same mentality that the in game livia one thing that people don't see when play games is how we don't suffer any  consequences or repercussions for the action we choose, so In game livia has Leon's Mentality one that don't have to worry for consequences or repercussions, But Livia is different she have to fear for them, when she slap julius she didn´t know that he is the prince and Let's remember when livia meet leon for the first time she was wary of him because he was a baron and a Noble, when she meet ange she was wary and afraid for herself in the presence of Duke's daughter, now you can Belive that Livia Can smile and be happy without Worries being Surrendered by Royalties and High Ranking Nobles? Obiously No, she is totally Scared for her life that she can't reject them for fear they will use their noble status to attack her or her family.  He Should Be More Mindful of the Worst Case Scenarios and how to deal with them instead of belive that there will be no problems because the plot says so
thankfully most of this problems are dealt with after this arc