I found the reason. It's because indeed you called her mistress at first but didn't submit the second time. But you maybe did that because you didn't want to get the feet scene. Anyway, this is the reason. If you want to try again (Just choose the first option during the second event)
I just did modifications that will come up still in 0.34 :
Arriving at day 40, When you will see the carriage, if you didn't kneel during the second event, you will not become her slave and you will get this sentence:
"You suffered the consequences and persisted in not bowing down the next time but she still forced you to obey her whims but not enough to make you her slave."
I made an exception. Only if you called her mistress at first and then decided to not submit, you will get a choice confirmation like that:
"But perhaps deep down you are still hesitant... Calling her mistress and then refusing to obey her..."
Choice
"Would you like to become [Bryanna]'s slave or not ?"
"Yes, I would like to submit to her and become her slave..."
"You finally feel inwardly like you need to submit to [Bryanna]... To become her slave..."
"No, I was hesitant at first but I didn't kneel for not becoming a slave!"
"You keep a cool head and decide to remain a free man"
Hope this will be enough to avoid that kind of situation again