Game is bugged, unfortunately. There's no way to talk to Melvin if you've already found him before. So, basically, it's a linear game where you have to follow the steps in the exact order that the dev planned, or else it gets softlocked.
Spoileres ahead. There are 2 ways of knowing where Melvin is. You can either use the quarters or the wistle. Using the unused method, if you haven´t already used both, might work.
I don't know if I'm talking about the same thing, I'm not sure I was softlocked. But the objective popped when it shouldn't, after i had already found Melvin. And that's the only way I could cross it from the list