Well my dh is a Jason (along with many many others born in the early 70s) and my FIL is an Alan. I don't think I've ever noticed anyone misnaming them. I guess you have to look at the relative popularity of the name, I'd be way more likely to mistakenly call a Jackson Jason than the other way around as to me Jason is very mainstream while Jackson is fairly unusual.
I don't really think that Jackson sounds that like Jason to be honest. The Alan confusion is even more surprising to me - Aaron and Alan start with a very different sound 'a'/apple and 'a'/air, while Ellen although a bit more similar is obviously a girl's name.
Forgetting names is not anything to do with the name though, that's just people being forgetful in my experience.