Diagnostic overshadowing
Definition
NICE Guideline NG96 - Care and support of people growing older with learning disabilities

In this guideline, this is used to mean the tendency to attribute all behavioural, emotional, physical and social issues to a person's learning disability or a pre-existing condition, while overlooking the possibility that they could be symptoms of other conditions or difficulties. An example would be attributing challenging behaviour to a learning disability when it could be a reaction to abdominal pain, which in turn might be symptomatic of a physical health problem.
Plain English definition
Think Local Act Personal (TLAP) Jargon Buster

When you have a particular health condition or disability and professionals use this as an explanation for all health problems or symptoms you have. It often affects people with an existing learning disability or mental health condition, and can mean that illnesses are missed or not treated properly.