Bug/Issue Journey of the Prairie King boss ran off the screen

I was playing the 5th level of Journey of the Prairie King for the 3rd or 4th time in a session (and in my game's history), and when the boss health marker got to the outer edge of the boss's barricade, the boss suddenly walked to the left non-stop, onto the black padding, and off of the window.

Here is the screenshot:

 

Hi, just wanted to add my two cents, this same thing just happened to me. I didn't get a screen capture, but the boss was nearly dead, then he started strafing left and shooting then just kept right on going right off the screen. I was a touch baffled, lol.