heres a better explanation:
hey, i reproduced it on your site — its not really a bug, its how overlay pages work.
your /bienvenue page is configured as an overlay in lay theme (in wp-admin you'll see "Overlay" settings for the page). any link pointing to an overlay page opens it as an overlay, which means the url stays where it was and both pages get layered on top of each other.
to fix: go to /bienvenue's page edit screen in wp-admin, find the "Overlay" metabox and turn off the overlay option. then links to bienvenue will navigate normally and update the url.
if you actually want bienvenue to stay as an overlay for some reason, thats a different setup discussion — let me know.
