Thank you for your answer. Here's the info I can give you.
How did I get her as my concubine : Her Spouse was one of my Warrior & Commander (I marry courtiers to bring useful people to my court like , I suppose, everybody does), he died in battle years later. As my wife turned into a drunkard, I murdered her. I took the concubine before the murder was carried out. I granted her the Poet Court position though she's poor at it but I have no other option. She does'nt have any children, never had. She is friend with my Chancellor who isn't landed. I previously noted that she had a very good opinion of my character and I thought she had the stewardship I wanted to expand my Domain. At the Time I try to marry her, she is not travelling and I just ended a War that I won and tried to mary here once back. My Idea was to use her Stewardship ability to keep one County more and the fact that after 45 there is few chances for her to get pregnant (I got three sons man, how am I gonna solve this damned succession ?)
I have a suspicion that your issue occurs due some specific aspects of character who is you concubine, hence the questions.
I think this is quite possible. But I'm only guessing. I didn't interact much with this character. Never had any event with her apart during the Seduction. I had two (the gift and then the decision to keep her as a lover).
Also, important facts : I forgot I was using a small mod wich is showing Character's Opinion and Personality, "Player opinion & Personality". Maybe it could interfere, I don't know. If that's the case, my bad, I genuinely forgot since it fits perfecty with the RP aspect of CK3.
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I also use a bugs fix patch you certainly have heard of.
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When I launched the game a few minutes ago, it said that the save we are talking about was created with a different version of the game wich doesn't make sense since I started it after the W&W and update were released. I use every CK3 DLC too (they're great by the way, though not coming fast enough

)
I don't have Time to let the game work and see what's happening then. I can do that later if this is needed.
Hope it helped a bit.