I've not seen that, the bug I'm referring to is to do with putting the mac to sleep when it's connected to the screen.
Oh, I think I’ve had that: it’s a total crapshoot as to whether it wakes? I thought it was a USB power problem for the longest time.
You can check the refresh rate as follows. Open System Preferences, then click on Displays. There's a radio button, titled "Scaled". Option-click that radio button, and you'll see a pull-down menu, titled "Refresh Rate". It should be 60 Hz or higher.
Sounds like for some reason the monitor is running at 30Hz. You can confirm that using an app like EasyRes.
System Preferences can also show this, check my other comment in this thread.