Thank you for your reply and the information.
For more context, I'm in an Excellent relationship with Shadowheart (but not romanced/flirting/dating), and she's been in my party since the beginning of the run.
Also, I haven’t romanced any other character.
I'm going to try the Poly mod and see how it goes.
I'll definitely give it another proper try in my second run, since my first playthrough is a co-op run currently in Act 3.
I'll come back with the results!
Thanks again!