The person whose name is missing from the credits still remembers doing the work.

Records of who did what are always incomplete. Names get left off for reasons ranging from oversight to politics, and the work itself is unaffected, since the person who did it keeps every hour of it in a way no list can remove. That is genuine and it is also insufficient, because credit is how work turns into future work. Private knowledge does not open the next door.

Say the missing name out loud when you can, in the room where it counts. If it is your name that is missing, write your own account of what you did while it is fresh.