Okay, so I'm having a perplexing problem. I am using the oxygen template. All of my pages are showing the "hamburger" mobile menu successfully except for the contact page:
I've noticed on every other page, there is NOT a line being loaded from an nucleus-ie9.css file. On the contact page, it does load that file.
There is a module there from RSJoomla. It is loading a form from their RS Forms Pro in a module position via a module. When I turn this off, it doesn't trigger the site to think it is loading as ie9.
I wanted to post this here as well and at RS Joomla in case you have any insights as to what part of the module might be triggering this to occur.