Thank you both for your reply! It is really helpful for a newbie like me to understand what's going one better. Agree with you both that it probably something with the rear elements of the lens. I cleaned the back of the lens and just tested with another roll with different aperture settings, hoping to find out exactly where the dark spot would appear with the f-number. And also hope that it's just some dust that went away after I cleaned it up. I really couldn't see any scratch on the rear lens, only saw two little dust within the lens, and they are not on the same spot where the dark spot is on the film.
I am still debating if I should keep the camera since I got it for only $300 (I think it was a bargain when the seller claimed everything seems to work properly). The seller is not a professional camera seller, just selling vintage stuff online, and I guess someone sold him this camera. So I am thinking if this spot would go away with cleanup or if it only shows with f22, I am wok with not use that setting going forward.
Curious what would be your thoughts on this if you were me? I really want to have a pocket camera that I can take everywhere with me, but only have a limited budget (no more than $500). Appreciate if you guys think there are better options within this budget!