Masking is putting on a face to _hide_ symptoms from others to appear more 'normal'. It's f-ing exhausting. The symptoms never go away.
In this case when you reduce symptoms with masking then you get different symptoms, like exhaustion.
And yes, this doesn’t seem healthy for the person who is masking, especially when taken to the extreme levels.
My biggest symptom as a kid was a difficulty to lie (white lie or otherwise) when asked directly about something. I did a year of therapy to learn that, and how I should learn it. The fact that I can now say 'what a beautiful dress' and not 'I guess it's okay, I don't really care' was directly taught, as do a lot of social stuff non-autistic people learn by mimetism. And now, I do learn by mimetism, I'm just proactive and conscious about it, I replay in my mind how charismatic people do stuff, and decide if I want to emulate them or not, then try to. That, masking and being conscious about my rabbit holes make social situations with 5+ person really tiree, really fast, but tiredness is my only symptom left.
The fist hint for me was the psychologist who taught me how to lie (or rather, explained to me social interactions, as i did not care about it at the time), and that's as clear as it can be, but sometimes its only weird result on IQ tests: if you moved countries if often don't mean anything, but if you did not and you scores are not homogeneous accross the tests, like 4 in the high 130 and one in the low 90s, you're probably on the spectrum too.
Even more - that is also evolutionary change, as 40% speed increase of brain development in embryos is exactly that. How other things are adapting is of different matter. All people will be autistic eventually.
ABA (masking) doesn't reverse autism. It just makes it less visible from outside (no matter the cost from within).