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