Separation Anxiety?

Yes, at least they avoid real/healthy/consistent intimacy versus fantasy bond-type intimacy. APs probably won't leave once attached, but getting attached in a genuine consistent and loving relationship that's healthy off the bat is less likely because that may feel strange, uncomfortable, boring. It wouldn't necessarily consciously feel that way, though, just sort of lacking chemistry/sparks because it wasn't overwhelming the nervous system through triggering.

Your take on this is correct. Understand what you seek to understand, but don't over-empathize or take on too much responsibility. As some have already posted, if you were aware, consistent, treating him well, you weren't actually responsible for causing the triggers. They existed before you came around, and it sounds like he hadn't learned how to properly communicate his feelings or to self-regulate. If someone can't do either of those things, or ideally both, you don't really have much of a shot at a healthy back and forth no matter what you do. I've written before about how I addressed my issues partway through my relationship with my FA ex, and while I always treated him well I became a lot more patient, stopped personalizing, and mostly got my own triggering under control, and my presenting as much more secure had exactly zero impact on his behavior. He repeated his cycles.

FA does have a high level different set of behaviors than AP, and getting triggered anxious can look the same, but there's different thoughts and motivations behind it. Your ex did test high FA, with a higher anxious mix than avoidant. He was likely scared to let his guard down and enjoy reuniting because of his own inability to trust the stability of the relationship and fear of losing it. When I reunited in an LTR, I was always thrilled at the beginning but would get moody after a day or two because I knew how much parting was going to hurt.

When I talked about feeling AP triggered by separation, I wasn't talking about my most recent relationship with my FA ex (which was not long distance at all). I was not overly AP with him, barely at all until his big deactivation, and didn't have much separation anxiety because we always had plans. However, I've had it in many prior situations. Usually, at the beginning of a new relationship where I really liked the guy and was unsure how long he'd stick around. Again, it was helpful for me to always know there would be a next set of plans to hang out when we parted, even if they were a ways off. That's all I needed to skip the anxiety. But anxiety was also triggered by what I wrote about earlier, mostly when my unexpressed expectations weren't met (like what I said about plans changing), or feeling a change in dynamic that felt like distance was growing. And I never dated securely attached men, so it always felt like I could lose their love at any minute because I COULD. I wasn't paranoid or overidealizing or misreading, but was choosing men who couldn't give stability and that made everything much worse.

I've done LTR only a couple times, and yes, for an unaware AP that was the worst. The first was a long period of geographic distance in the middle of a long term relationship, and it was horrible. The guy was either a high anxiety FA comorbid with mental illness or a covert narcissist, it was almost decades ago so I don't know what he is, but we were a codependent mess wondering why the universe conspired to keep two people so in love apart. Highest highs, lowest lows, ridiculous obsessive communication in a time that technology was not like it is now. I've had a couple other sticking my toe in LTR situations since then, but basically won't do it. Although now that I'm earned secure, I probably could without it being a dramatic or painful roller coaster.

FA strong in AP here. For me, there are a few different types of anxiety and I don't know if separation anxiety is necessarily the best term. Sure, I could start feeling anxious in an AP way if I don't know when I will see the person again or something triggers that like them acting avoidant. But I might also feel anxious over the fact that I have feelings for them. And I might also feel anxiety without a conscious cause, like maybe I only realize it's even connected to the person because I know so much about myself and attachment theory. And I might also feel a deactivation, where I start to doubt my feelings for the person and that makes me anxious, worrying something is wrong with me or I will never find love.

mmmm i definitely can identify with that. i couldn't see how our future would develop if we didn't plan for it actively and made agreements to make it happen. allowing things to develop naturally made me feel like he was wasting my time - keeping me around without promising me a future.