I feel like you just described me and my ex. I had something very very similar, with a slight difference. Welll, for one, I'm not ace. And also the scenario was different, as we had her ex in the picture, them being best friends and flatmates. Looking back, I guess he still loved her.
I don't know.
I can be so oblivious when it comes to romantic feelings.
I tend to interpret everything as a platonic gesture cos that is my only frame of reference.
So, she was so paranoid that I'd get jealous, but strangely, he did not bother me at all. I was glad she had somebody she trusted, and that she had a good friend. It was so natural, I wasn't even sure if I was the third wheel or him, as far as I was concerned it was a perfectly functional tricycle, though I wasn't dating him or having anything physical with him.
But absurdly, I felt guilty that I did not feel possessive or territorial at all. I mean according to my allo friend, I was supposed to hate him or something to prove that I loved my girlfriend. Wut?
Uhm, it was a surreal experience. Funny thing, after years, I miss them both.