It is definitively not caused by selection bias. Given the fact that you're describing it as "most likely autistic" and the fact that you're here and present on social media and advocating for yourself, you would probably fall into LSN or Level 1 autism if that is the case. The degree and severity of muscle mass issues are least observed in Level 1 and most observed in Level 3.

Muscle mass is not directly used as a diagnostic criteria of autism or ADHD, so they aren't just selecting based on muscle mass. People of all neurotypes have all sorts of bodies, so while it can be diagnostically helpful as a clue it's not what doctors are looking for.

This effect is observed in all parts of the population, including those who are too young or noncommunicative to be aware of the effect, so it's also not some sort of self-fulfilling prophecy. We don't know 100% why it happens but it's probably sensory related at in early childhood and then becomes habituated as well.