I have found a work-around without having to close the client, even though it shouldn't be necessary (read: the game should accommodate for players with multiple monitors, even if the game doesn't run on multiple monitors)
Note: I generally have this issue when I set my Windowed Mode to Borderless in the Options menu. If it's set to Fullscreen, I never have encountered this...however, the steps below should be valid as a workaround regardless.
1) Unfortunately, you must wait until the match loads...I have not found a way to get to the Options window from the load screen. Once the match loads, press ESC to get to the Options menu.
2) Then, move the Options window to the second monitor (this step is only necessary if you cannot see the Back button at the bottom of the Options window).
3) Click Video and change the Windowed Mode to Windowed and click Back. That should move the entire game to your primary monitor.
4) Then you can change the type to Borderless if you want to. But just remember, this will most likely cause you to repeat all these steps each and every time you start a new game.
The issue is still something the developers should be working on, but at least we have a way to resolve without re-launching the client.