Why the dynamic role isn't getting assigned to intended users in PeopleSoft

I was once working on a project where I had to create Query for dynamic role assignment. Everything was going well as I was able to create it without any challenge. The query was fetching the desired set of users to whom the role was supposed to be assigned.

However, when I ran the Dynamic Role Assignment app engine (DYNROLE_PUBL), the role wasn't getting assigned to the intended users. I did the RCA and turned out that the PS Query which I created, wasn't returning the distinct OPRIDs hence the app engine process wasn't assigning the role to users.

To resolve the issue, I added a DISTINCT keyword next to SELECT using expression in the PS Query which though started returning distinct rows yet the main issue remained unresolved. I ended up spending several hours to get around this but no luck.

Then I read somewhere that instead of using DISTINCT keyword, I should rather enable distinct option in the query properties to get the distinct rows. The option is shown below:

Then I ran the process again and the issue got resolved.