I'm a discovery writer (no plan!) and I seldom have this problem. I would suggest actually writing less on the plan.
The issue, which discovery writing takes care of automatically, is that main characters tend to grow throughout the book. That is what happened to you, in your own "discovery" phase of writing (some scene) you embellished and created some personality traits.
That is a good thing, a natural thing, and readers find it entertaining to gradually discover character traits. But it is very difficult to plan out in an outline. Many plotters have the problem of their characters growing and then straining against the leash later in the story, they don't want to say and do what you planned for them to say and do. It is no longer in their organically developed character, and feels awkward, or cardboard, or forced, when the time comes to write it. That is what happened to you.
The way around it, if you feel like you must have a plot, is to skeletonize the plot. Do not put so much detail in it. Write only the turning points, things you can remember while actually writing the early parts. This will leave you the room you need to improvise new character points while writing, and to then improvise when you get to the scenes.
1) The team decides they have to go to Chicago.
2) In Chicago, Mike hooks up with Alice. Consensual, but she regrets it. He knows it.
For (1) In your head, from the beginning, you know they have to decide to go to Chicago, but don't write in all the emotions or dialogue of WHY, leave the character arguments and motivations for when the time comes.
For (2), From page 1 you know Mike and Alice have to hook up; consensually. So you won't give either of them a trait that would sabotage that hook up or make it implausible. But wait to write the emotions and motivations until the time comes. At that time, you may also go back and insert incidents that foreshadow the event, or make it more plausible.
That is how I work as a discovery writer, there are big things I do know about the relationships but I intentionally develop the characters as we go along, by letting them do and say whatever comes into their head. Or to be more exact, they do, think and say what I think that character-so-far would most likely do, think or say in the developing situation.
So my recommendation is to write less in the outline, stick to the bare bones of plot turning points, and let your characters become people in your mind. There are many ways to decide you have to go to Chicago. Some characters may like the idea, others may hate it. Good, that creates conflict and tension.
There are many ways for Alice to have a disappointing sexual experience with Mike. That could be Mike's fault, or Alice's, or the environment or demands on their attention making it a rushed experience which did nothing for her.
You know when you are writing that is how it has to be to serve the plot, let the characters figure it out when you get there.