How about this - a PTSD sufferer starts needing to isolate. Doesn't tell partner anything, just leaves. IF the sufferer knew before hand that isolation might be an issue in the future (IE, it's happened before), then yes, they might want to let their partner know it might happen, and not to take it personally. Almost impossible to do, because a relationship is about two people. It's hard not to take it personally. But being warned ahead of time at least makes it less of a mystery.
If it's the first time, sufferer had no idea, I would imagine that's a different situation. At that point, PTSD partner disappears, I'm pretty sure it's less an issue of "shit he left me," and more of a "shit, he's missing, call someone to help find him." Once the emergency (because a missing person is) is over, THEN it's time to figure it out. Or, if PTSD is a known issue (and supporter did their homework), they might tell themselves "Shit. Isolating. I hope he's ok." And then figure out how to deal with their OWN shit, on, oh, say, a forum like this. If/when PTSD partner comes home, THEN the talks can happen. There is also a difference between a flashback, where the sufferer doesn't even know what they're doing (more a "missing person" scenario), and "just" isolating, where they can't deal with any more stress, good or bad, but are still able to function (and go to work, and talk to other people, just not their partners).
When PTSD is new - no one knows what to expect. Those of us who are sympathetic know that. Hell, when PTSD isn't new, no one is still quite sure what to expect.
I guess my point is this - PTSD tells you everyone else is a danger. If you can't get past that, to the point where you can at least WARN your partner that you have it, and they need to research it, and here's what you've been dealing with that may affect the other person...you may need to figure out if you're really ready for a relationship. Because, seriously, relationships involve two people (or more, hey), both (all) of whom need to feel safe. If there is any hope for a relationship to work out, communication has to happen. And if PTSD (or its diagnosis) is new, in an already established relationship, trust may be physically impossible for the sufferer, and I'm not sure I HAVE an answer for that, beyond to keep communicating. BOTH people have to do what they can to learn everything they can, and figure out what to do. And, yes, the supporter needs to figure out if they can be in a relationship that may be one-sided a lot of the time. That's on us, as supporters.
I'm sure now I have to touch on "casual" relationships/dating and the human contact we all need. Be clear from the get go. You don't need to divulge your medical history, but do warn them that you aren't looking for anything serious. And if you think you might be getting serious, start communicating. Be firm with your OWN boundaries, too. If you don't want, or can't do, all the scary shit, like trust and emotions and love and anger and happiness, that goes with a relationship, then don't do it. And if you think you want to try, then be honest with your new partner.
I guess my overall point is this: A relationship without trust, is almost impossible and will require more work than I think any of us realize. It's physically impossible for you to trust someone? If you want to have a relationship, you need to figure out how to account for that. If you can't do that, then, no, don't expect any sort of good or healthy relationship to be available to you. Sufferers push those closest to them away the hardest, because we are the most "dangerous." If you're in a place that you recognize that, then you need to account for that. We supporters know it, and in an ideal world, won't need a text to tell us you're still out there; but it's not an ideal world and there are such things as, oh, car accidents. If you're in a place where you just need to be alone, we will understand. If you disappear entirely and physically can't reach out, I'm guessing your PTSD isn't as under control as you think, and we'll figure it out when you come back to us, after we verify you aren't dead in a ditch.
We, as supporters, are generally more than happy to figure out how to pull twice our weight to make things work. We forgive things that in a "normal" relationship shouldn't be forgiven, sometimes while knowing it could end just like that because our beloved's amygdala was hijacked again, and there's not a damn thing we can do about it. For "normal" relationships, love can be enough. In PTSD relationships? Love just isn't enough.
To address the OP: The confusion I see on these forums comes from people who are new to the PTSD gig - either because the relationship is new, or the PTSD is new, or the symptom is new, or even the diagnosis is new. There is pain as well, but a lot of confusion. For those of us who have, one way or another, been living with it, we come here to have someplace to vent. We want to understand, we don't want our loved ones to hurt. And, hey, maybe there are some who just haven't done their research. Or we have, and the symptoms are so shocking, we didn't know to expect THAT. So we come here to see if it's normal, and to talk about how it hurts, because we can't talk to our sufferers about it.