Its revealed that her love interest is being controlled by some godlike force and, despite the fact that he actually loves her and appreciates her, he is forced to say and do the super cold things against his will. He doesn't actually love mc's love rival: When she shows up, it's revealed that he's actually constantly thinking about Ilya and he suffers because he can't control his actions. Combined with how terrible life always ends up for him after mc dies (and before we always felt good about this), it generates amazing pathos for both mc and her previously thought to be unobtainable guy. If you're like me, you think this is great, super interesting, a massive bombshell, and want to see mc get rewarded for her efforts by somehow defeating this force and winning the guy who has always loved her, right?
Jokes on you! Almost immedistely after this is revealed, the story takes a break to focuse on runner up love interest's who was previously a demon Candidate based on the heartless things he would say. The author will try and retcon his aloof heartlessness via flashbacks: "Nooooo reader, you had it all wrong! He was the most caring guy ever and his comments were misunderetood!" Fine, basically the same thing was done with Ilya's love interest when its revealed he's being controlled, right? Well, they dont do this via one chapter. Worst of all, the author decides to focus on this dweeb by spending multiple(felt like at lesst ten) chapters flashbacking about his pointless [life in the age of magic. Turns out this guy already had a love life before wanting to reward our mc with his "affeftion" (author even says its basically pure pity) and was already fulfilled before he became immortal. Our mc is essentially the 2nd wife if that is indeed the ending.
The cool bit about her love interest being controlled? Completely forgotten even though you can start to see bits and pieces of him struggling to fight the control break through his personality even at the end.