To answer the topic question, for me the big difference could be the age groups. Your talking about mostly teenagers or people in their early 20’s with Accutane, vs closer to 30s im guessing on average with Propecia use. It could be alot easier to discern changes in the Propecia group as opposed to everything a young adolescent/adult is already going through in general that also happened to take Accutane, To make all the connections.
Not only that there can be a progressiveness of side effects that worsen with time. so now 20 years later as someone is approaching their mid 30s, do you still associate everything with the drug you took 20 years ago? its more of a “what if” I never took Accutane then PAS as the lines becomes blurred.
Take this study for example,
Severe Acne in Female Patients Treated with Isotretinoin is associated with
Dysbiosis and its Consequences
**Isotretinoin is associated with
dysbiosis, malabsorption (with visceral fat decrease) and signs of
dysimmunity. Symptoms develop more than 20 years after the intake
of the medication taken at usual range and duration