User is not available in the User mention list.

[andrew sutton]

One answer seems to be missing from the suggested problems.

This is that the user needing the mention does not have the permission to see the project.

In Project > Admin > Permissions, ensure that the browse Project permission contains something that the missing user mention is within.

[Deleted user]

As you mentioned, check if that user has the "Browse Project" permission according to your project permission scheme. It maybe tied to userID/role/group.