Cant customize menu?

Hi.
I have kinda strange problem on t3_bs3_blank template
Lets say I need to make module position in menu bar.
I know how to do it, and all is ok.
But if I set this new div not to be visible on smaller screens, bootstrap menu gets broken. When expand button is clicked, nothing happens.
This happens if I hide that position with css display:none and if I use visible-lg directly in code also
Is this a bug or I am doing something wrong?
Unfortunately cant give link to website.
Thank you