Very few people can resist tsunderes, it seems . Though I think it also helps that we get a peek into Ava/Adam's thoughts at certain points and it shows that they're not as egotistical as their first impression makes them seem, and that they're at least partially willing to (privately) admit when they go too far and show awareness when they are in the wrong and that the MC wasn't the cause of some problems. I can imagine from Adam/Ava's thoughts alone that they're going to, being the type to show anger when really worried about a MC they are close to.
I find it funny to see a good deal have chosen subtle/shy. But then again, it's nice to enjoy being pursued rather than always having to do the pursuing, especially when it feels more in character for some MCs to let another take the lead certain time or be shy at certain moments with certain characters. Don't get me wrong, it's fun to catch some characters off guard or possibly make them flustered a bit, but it also feels interesting to see how a RO goes about their own courting when they're interested enough in the MC. I noticed that most video games do tend to go the route of having the MC being the initiator, which can feel a little much at times.
Like, I could see Lupin being more likely to flirt openly with Nat while they are alone together due to how comfortable and at ease he is around her, though his slowness to trust makes him more inclined to take a more subtle approach most of the time (just that being alone with Nat would boost his boldness at times). But in a situation with someone like Manson, who he spends most of his time being snarky with, sudden, unexpected close contact with him would easily get Lupin flustered. I like to think he freezes up partially since Manson is a bit taller than him and partially because he genuinely has no idea how to react to sudden contact with someone who he spends most of his time disagreeing with (though not in the same way confrontational way he does with Ava) and who isn't a person he feels as close to as Nat and Felix.
You probably already figured this out by now, but I just realized that Lupin actually did get a few points in friendship and flirt with Manson/Morgan in my play-through. When I replayed, with the choices I made around him, it seems he responds to not backing down to him at certain moments and being snarky with him. That said, if you do end up acting shy/subtle in certain moments, it's like some hunting instinct suddenly triggers in Manson/Morgan that causes them to pursue when Detective MC tries to shy away.