Just a different take on the "paying no attention" thing...
The few times I was in a situation where both Hubby and one of my guys were present, I found it very difficult to sort out where to give my attention, and for me that brought up some anxiety. I didn't want to ignore either of them, nor did I want one to feel that I was paying more attention to the other. Bless Hubby... he understood this and found subtle ways to show me he was okay with me paying more attention to my boyfriend than to him, because Hubby was the one going home with me at the end of the event.
And those were *planned* things.
If I was with one partner and accidentally ran into another, I think my first reaction would be "Oh, crap, they're both here, what do I do?" Even though I always have complete openness with my partners about who else is in my life and have nothing to hide, sometimes anxiety interferes with my being logical in unexpected situations (or even expected ones). And if it was a case where Partner 1 and I were just walking past Partner 2, it might not occur to me to say anything or even wave to Partner 2 until we've already gone by, because I would be trying to navigate that initial reaction.
So to me, the woman not acknowledging you when she's with her other guy isn't necessarily a red flag or an indication that she's hiding you or being dishonest with him... it might just mean she isn't comfortable with interacting with one partner when she's spending time with another, especially when it's an accidental encounter. The only way you're going to know for sure is to ask her.