It seems that after x amount of time (somewhere between 30-60 mins?) of inactivity on a page the session is being destroyed/cleared/etc.
I will sit on a page with an active session for an hour and make 0 calls to the PHP. The next time I make a call to my PHP the session is completely gone. I have 0 variables in my session. I'm logging in the beforeAction and the session is just an empty array.
I thought it was my custom code that was clearing the session accidentally but before my code even executes the session is gone which leads me to believe it has something to do with Craft bootstrapping up to receive the PHP request.