How to reproduce : 

1/ Create a project with title "Project1" . 

When created, project is visible in tab site of cms/_admin. 

Title of created project is "Espace projet - Project1". 

Note : Following configuration is used : even if prefix is empty, "Espace projet - " is added. 

2/ Go to Update project page by clicking 

3 / Project page is opened. Click only on "Validate" button without doing updates. 

4/ Open cms/_admin => Name of project has been updated, it is now named "Project1"

(Prefix "Espace projet -" has been removed.)