Hi,
Its because the parent wrap is floated and floats are shrink to fit causing the last item to drop to a new line (an old bug that seems to have returned in ie).
You may be able to cure it by using flex instead.
e.g.
#navigation{
display: flex !important;
}
You will need to test though and remove the flex when the mobile menu kicks in.
Edit::
Actually that will be too awkward I think. Try this first.
#navigation::after, #navigation::before {
display: none;
}