Thank you for your reply! This is not due to orphan users but how Management Studio works (see my previous reply)

One of my colleagues found that it is because this user is member of another role, which is a member of 'db_datareader' role.

Lesson learnt: SQL Management Studio displays as role...

I have the exact same problem.

I removed some users from several roles but they still appear in the list of role members in SQL Management Studio.

The user '3' was removed...