I've recently come into watching for this behavior in INFPs and wanted to extend the observation to all of you to get some more input.
From my experiences I've had first-hand or been around in my personal life, I've started thinking that INFPs may have a propensity for managing (or not managing) their feelings and self-awareness in such a way that while in a relationship, they are not communicative and thus solution-oriented about their dissatisfactions. And that by the time they finally commit to their dissatisfaction and make it known, it is on their way out the door, leaving their partner with no say, no hope, no relevance.
A little more info thus far would be that I've seen this happen twice with INFJ/INFP relationships and of course when I say "not communicative" that is coming from an INFJs standards. The INFPs seem to harbor a mentality along the lines of that if something is wrong, they "shouldn't have to" point it out. To me that just looks lazy and self-defeating of the INFP.
If this is a pretty habitual way of handling problems, communication and relationships for INFPs, then I have to say that they sound like really unreliable lovers. And that they will move on completely, emotionally, without your having any idea of it seems outright inhumane to me. Again, though, I am an INFJ.
So! Somebody call me an idiot and somebody tell me that I'm right? I'm really curious as to whether other people can and have observed this, and would love any opinions on how and why this may be.. especially if I can view this demeanor as simply being different, rather than really lousy.