Spoiler, the duke runs frantically through the snowy mountains to find her after her escape attempt, and when he sees her lying half-covered in the snow, he stares for a while then said:

"Were you lost?"

"... ... Yes?"

"You got lost."

He repeated the same words like a spell. As if he was telling himself that he wanted to believe that.

I stared blankly at him. His expression was not well visible in the backlight.

"Tell me you're lost, Leonie."

He knows why she ran away and that she won't stop, but he just wants to desperately deny it so as to not break the peace. Kind of like convincing yourself not to see what happened because you'd rather erase that fact and live as usual.

The duke... has his circumstances but he isn't unsociable and isn't cruel.

Or course he shouldn't hold someone captive and drink their blood so regularly. But he listens to Leonie's complaints and compromises on the blood amount and fixes her situation.

He's not an awful person. You can see that he doesn't wish to drive Leonie into a corner either, he just can't let her go, and that's all Leonie wants.

When she wants a good position where she can protect herself, he randomly makes her a secretary, leaves her in a dark and small room to achieve nothing. Even after they made a deal, he sucks her blood right from the neck and then throws her into the dungeon right after...

The running away scene... He really took his sweet time saving her. Bro, she is going to drop dead in the middle of your long looks and calm conversation...

So basically the ML teams up with og!FL to take down the prince, but decides to not tell FL anything. You can sniff what’s coming from a mile away. But the thing is she is repeatedly demeaned and humiliated because she wasn’t let in on this plan. After all the despair she went through, the ML didn’t do enough groveling for my taste to make up for it. Like he gives her the freedom she wants, but he doesn’t prove enough to me that he regrets the turmoil he put her through, heck I don’t think he even understands the turmoil he put her through, all that she was willing to do for him, he doesn’t prove to me that he deserves her

All she's doing is milling around, fretting, telling lies, forgiving the person who is killing her (even though the person said outright at one point that it's being done on purpose), and making everyone's lives hell. I know this is a common trope but it's reaching some absurd extremes here.

And, the supposedly sharp team of people dedicated to keeping her alive don't notice anything strange about the new visitor growing in strength and health as the FL weakens? Nor do they pick up on the FL -constantly- asking when this person will leave? And see no need to relocate said person to another vassal's castle in order to keep the Duke's secrets??

They just cleaned out one set of the second Prince's spies who had infiltrated the castle but, no, let's keep this person here and not question them.