I have tested your theory and it seems like you are right. I started new game and chose to touch Mir flower. This gave her some sub points for me, and she starts as more sub char than others I believe. Bcuz its the start of the game, I dont have much frendship with her and she dont have much drive, romance, sexual tenshion. So I tried to talk with her, build up her submission durning conversation and yes, she asked for sex with me on top.The character's baseline personality (their "drives") matters a lot to decide if they will ask for sex where you are dominating them. Claw and Nash are probably never going to do it early in the game, Ate is also unlikely, Val might be possible (although I don't remember seeing it), Mir is the only one that starts somewhat "submissive-leaning".
I don't know the exact mechanics, they are very involved and you'd probably have to ask the dev, but for the purposes of the actions a character will take out of their own volition, you are probably looking at 3 sets of stats, each having a different weight for different type of "decisions", I think for things like asking for sex (and what type of sex they will ask for), they might be weighted like this:
Their personal drives > Their relationship levels with you > Their current mood (the bars you see in a conversation)
What is possibly happening in the examples you mentioned is that, while the characters are submissive towards you, they are probably also considerably affectionate towards you (high friendship, high romance), which might be making them be even more likely to prefer just asking for egalitarian sex, due to their "love" drive. What I mean is that when a character is choosing their "goal of the day", them having a good relationship with you and a high love drive makes them more likely to try to get intimate with you in the way they prefer (egalitarian for Mir, mix of egalitarian and dominating for Nash), rather than randomly suggesting sex where they are your submissive.
I don't know how feasible it would be, but if you could get their submissiveness high while keeping romance and friendship relatively low, they might be more likely to ask for submissive sex, but they might also just avoid you entirely and go get involved with a different character, I don't know.
Maybe there is more for that than just sole sobmission > frendship, but how are you gonna build just submission? Then you have either dominant or equal sex, partner will receive sub points based on your actions, but you and her also acquire romance and drive point.
So in the end you are bound to be asked for equal sex from chars even though you are making them as sub as possible?