Ok so it was a mission called Defend Starlight Drive-In. The mission randomizes the settlement location. Mine happened to be Starlight Drive-In. You have to defend the settlement from attacking super mutants. After I killed all the super mutants, I then had to go talk to Preston Garvey to let him know the job is done. When I complete the mission, it shows all the tasks checked off except for the talk to Preston Garvey one. Is it a glitch? It's not big glitch at all, but I like to see all my completed tasks checked off. Did anyone else have this happen to them?

You could have talkee to him about something else. I normally have a few things to hand into him. However, the chat dialogue gets funky.