I have seen that when there is a database, and the user has been created but login has not yet been created.

(Not sure if that also happens when login exists, but instead of being mapped, a user is directly created in the database)

Can you check if the user in the database is disabled?

If it is - you may have to drop the user, I think it will also require changing owner of the schema the user is associated to - to dbo.

Then map the login to the database. It could be something else too, but sounded like what I'd seen.