Sud menu width

I am using twentysixteen on my website and I have a little bug with the submenu width:
https://www.wp-formation.com/

It appears recently, but I don’t know when…
I have corrected it with some CSS
.main-navigation ul ul a {
width: 13.6875em;
}

Otherwhise it looks like that
https://www.wp-formation.com/wp-content/uploads/2015/10/Formation_tutoriel_WordPress_4___Gd6d_organisme_formation.jpg

Thanks!