I've had this problem with two monitors and a small handful of games. It's basically syncing at half refresh. I think it's a Windows 10 issue but yeah, happened a few times.
Fallout 4 was the worst because i could not force change it without it acting all weird.
I remember reading something about DP @ 60hz being made up of two 30hz signals and that is where it gets the 30hz sync from. However it also happened over HDMI.
I wouldn't worry too much, I'm sure it got fixed over time. Also, just make sure you've installed any drivers for your monitor and under your actual GPU properties in Device Manager it's set to 59hz or 60hz whatever is showing there.
Oh yeah... Go into Nvidia control panel and set Adaptive Vsync globally. This in itself should take care of your tearing/smoothness issue.