If you scroll down to the bottom of that page that you took a screenshot from, are you able to see the User resource, or does it say that it is server-scoped?
Cool! This user was likely accidentally added a project-scoped user then, whcih is why they are unable to see the project in the project list. You can go ahead and safely remove this user and re-add them using the instructions here: https://www.medplum.com/docs/user-management#removing-memberships