I'm on the low end of whatever scale measures compassion, I don't know if it's my very pragmatic upbringing or just a feature of my emotional landscape, but I'm pretty mercenary with who I spend my energy worrying about. I take care of myself and those I feel responsible to- and on some level the rest of the world can burn.

As to your question, I find that unlikely to happen. We practice pretty thorough openness in our communications- both in the practical and tangible details of our lives as well as the fuzzier details like emotion. I don't need to have interaction with my metamour to know what is going on in the relationship they have with my partners. In fact, more often that not I know intimate details about feelings and desires before the metamour does. Part of that is the regular and substantive communication, but a big part of that is that most of my serious partners (and a few of the more casual ones) ask me for relationship advice on a fairly regular basis.

I'm no therapist or relationship expert, but I made a lot of mistakes in my youth- and I mean A LOT of mistakes. My relationship history reads like a trashy dime novel, the farther back you go towards college and high school the uglier it gets too. So the people who know me intimately enough tend to run their feelings and thoughts about relationships by me, simply for a "Hey, how much is this likely to fuck things up" check. Because if there's a way to fuck something up, I probably did it at some point or another.

But even without that component- if your relationship has the trust, honesty, and open communication that are hallmarks of a healthy relationship (poly or otherwise), then you shouldn't ever have to worry about unpleasant surprises involving metamours. It doesn't matter if you're communicating with them or not, as long as you have healthy communication with the adjoining partner.