Chronic halitosis despite good oral hygiene, non smoker, no strong smelling foods being consumed, healthy weight range and no other issues. causes?

Sinus problems: Sinus congestion or low grade infections can cause bad breath. It would be good to have an ENT doctor look at your mouth and check your sinuses.

Other sources: You may have sinus problems, or stomach problems. Don't limit halitosis to just your mouth. That is just where the odor is coming from. Although many times is a dental or hygiene problem.