[bug] Navigation sub-menu does not pop out on hover

One of my clients has a MMD navigation with up to 4 levels. They noticed that when the mouse hovers over a 2nd level menu with several sub menus, the first sub menu does not open on hover, but all others do. To open the first sub menu, they need to click. 

 

In addition to that I found that if you hover over the second sub menu, it pops up, then you can hover over the first sub menu and it also pops up without clicking.

 

That looks like a bug to me.

 

Ideas?