I haven't seen the report on this so I may change my views on further information, but until then
First of all, many adult children have all sorts of health needs and parents wouldn't necessarily expect to see their child between 4 months, who may live in their own house, have a job and have their own kids, live in this country or another.
Learning disability/needs. Presumably again the parents expected their son and he expected to be living independently, so were not necessarily being neglectful because they hadn't seen him for 4 months. I am sure my parents haven't always seen me every 4 months, not when I was young and busy. and believed I was ok when I told them I was. Parents who have children with learning needs are encouraged to encourage their child to be as independent as possible, to develop their full potential.