Hey @varunharidas ,
I think that's expected. As you can probably imagine, I think our search sees the exact match on "sun" and so scores the document really well.
I'm not sure if there's any way to work around it. If you wanted to, you could probably apply a value boost on the "name" field for "Joseph Sunny", or even create a curation for "joseph sun".
Are you using this for an autocomplete? If so, you could use Query Suggestions which would probably give you the behavior you are looking for.
Otherwise, in my opinion I'd just leave this as is. At the end of the day, do you think this would cause a user to not find what they're looking for?