Simon T.:
It could very well be that. Shinzen Young talk about depersonalization/derealization in one video. It's supposed to be found somewhere in the classic text being described as "falling into the pit of the void". It definitely feel like falling into the void. It doesn't last very long for me, maybe 2-3 seconds. I ended up crying quite a bit during the rest of the day so there was definitely a strong emotional component hanging around.
What's tricky about all of this is that there are a lot of psychological states that have similar characteristics to experiences described in contemplative traditions. Although there may be some similarities, I wouldn't say that there always the same. Sometimes they are really quite different.
An example of this is the feeling of "emptiness" felt by individuals with Narcissistic Personality Disorder or Borderline Personality Disorder. Someon with BPD may feel empty inside, and then relate to the texts that speak about Emptiness or the Void feel like they have an intuitive understanding of the teachings. But this is not the same Emptiness that the great sages write about and teach.
Dissociation/depersonalization/derealization (pick your favorite) can be similar. Yes, realization of not-self could be seen as "depersonalization" in some contexts. But the term means different things to mental health professionals and meditation teahcers (unless one person does both, which is common these days. Such people should know the difference).
Not that you're asking for advice, but I sitll recommended what I suggested in my first reply. You void experience sounds more like the workings of an unconscious defense mechanism rather than an opening into realization, or even the shift into a stage of insight. It doesn't have to be a big deal, and the reaction of your mind can be reconditioned. Simply and gently bring yourself back to a wakeful, integrated state when this happens. Relax, pay attention to something tangible in your body, like the temperature of your finger tips. Once you regain your composure, return to your practice in a kind and gentle way. Over time, your mind will stop finding anxiety so scary, and will stop trying to protect you from it.
Also, feel free to get a second opinion ;-)
Best!
-Jackson