I don't have a welsh accent even though I "should" -- after all, I spent all of my childhood there, except my first 3 years.
It's quite common for people on the spectrum to have an accent that's different to the one they were brought up with. While typical people adopt the accent of their peers (more or less), it's my bet that we're more likely to copy our parents or even people on TV, because as kids, we spent less time socialising with our peers than typical children.
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"Never try to teach a pig to sing. It wastes your time and annoys the pig."