Menu issue after upgrading to 4.4.9

In v.4.4.9 (under WP 4.5.2) the main menu subitems are all collapsed, and on hover they tend to be displayed on top of the collapsed ones - very peculiar behavior. I tried with a brand new menu - the moment it has submenus, they are all displayed hanging sadly underneath the menu bar. Any ideas on how to fix this? Thank you.