I have these episodes of dissociation, too, and I've spoken to quite a few other autistic people who have them. It's certainly not unique to autism; there are a variety of conditions in which they're common, and they can happen to anybody at times of extreme stress. I suspect that it is more common for autistic people, but maybe still only a minority of us.
I'd say that my episodes are not always the same; sometimes they are very much like jimmy m described, allowing my conscious mind to work on a problem, procrastinate, fantasise, etc. while one or more "auto-pilots" perform more menial tasks without conscious intervention. At other time, I can only describe it as my mind simply "going blank" for a little while, but with enough residual awareness that it is distinct from sleep - maybe a kind of catatonia? Even when I'm relatively alert, I experience spells of derealisation, where the world around me seems uncanny, as if I'm not quite really present in it.
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