I noticed I don't write or think a lot about them realizing feelings before everyone splits up. But that's also the only time they actually have within canon to act on anything… and it's an interesting angle to take, too, since in continuities where Leon has always been under duress, he has solid reasons to hold feelings back.
For one, Marian still comes first, whether he loves someone else or not. And it'd probably take until the last minute together before Stahn leaves for him to ask for help, if ever. So he'd be acting according to what he's sure is already going to happen: He's going to save Marian alone, and he's going to wait for the end of the world with her. There's no future for him with Stahn because there is no future, period.
For two, Stahn would just become another weakness. Something Hugo could capture and threaten. This Leon already knows how powerful Hugo is, so he knows there's no point in trying to fight him. He'd only be putting Stahn in danger if he let himself get close to him.
But I think both of those could cancel each-other out, too. The world is going to end anyway. He's already going to help Hugo destroy it, because he already has the ultimate bargaining chip against him. There'd be no reason for Hugo to take Stahn as hostage too; it'd be redundant. So why not? Why not indulge while there's still time?
The only reasons I could think of for not doing so would be not wanting to have the inevitable fallout after Stahn learns what he's done, or a subconscious hope that Stahn will stop him if he's free.
(I ended up trying to write a oneshot about this. Key word "try".)
(Why do I keep saying Hugo when I know he's possessed? idk, it just feels more right to refer to him as Hugo when it's from the perspective of a character who currently doesn't know he's possessed.)