Do all people who have autism have a special subject

I don't at the moment. As a kid I collected electrical junk, then in my late teens I was a music geek, then it was cameras and music technology, but in between, and for the last few years, I haven't had any special interests. Like a lot of other autistic traits, they come and go.

I don't know, think there are times when I'm happier and more relaxed and then the traits recede a bit - but they're pretty present most of the time.

I probably didn’t explain myself well enough there. Certain core traits like poor social communication; fixed thinking, obsessive interests don’t fluctuate, they are always there but I find that some stuff like being convinced that someone knows what I’m thinking without me telling them or believing that others can hear my thoughts because they’re so intense or being completely unable to realise that someone else could possibly think/feel differently about something to how I think/feel, that does fluctuate, usually dependant on context and what is going on in my head at that time. At the moment those ideas seem silly to me but in other phases they can get quite intense. Don’t know if that’s just me or if anyone else experiences that?

I probably didn’t explain myself well enough there. Certain core traits like poor social communication; fixed thinking, obsessive interests don’t fluctuate, they are always there but I find that some stuff like being convinced that someone knows what I’m thinking without me telling them or believing that others can hear my thoughts because they’re so intense or being completely unable to realise that someone else could possibly think/feel differently about something to how I think/feel, that does fluctuate, usually dependant on context and what is going on in my head at that time. At the moment those ideas seem silly to me but in other phases they can get quite intense. Don’t know if that’s just me or if anyone else experiences that?

but I get more and more 'absent' from the world as I get more stressed and unable to compensate or rationalise.

I can identify with that.... it is like a self preservation "kicks in" - in order to maintain a certain level of control or balance.  I remove myself and become a mere observer if I can, or completely internalise as a strategy.

Not sure, I think it's different from day to day depending on my stress levels? Is that what you mean by fluctuating? I'm not sure they're ideas with me that I focus on rather than just a baseline assumption which I find it hard to step outside of. I can do it by rational operation if I have headspace but I get more and more 'absent' from the world as I get more stressed and unable to compensate or rationalise.