The DMB command sets the Display Mode of the display. This determines how pixels in the next commands interact with existing pixels on the screen.

The manual specifies four modes (NOT, XOR, OR and AND).  I contacted digole to find out how to return to the default mode where text writes over the top of other text. They advised there is another setting. "C" for Copy. 

DMC will reset the display mode to its default setting - allowing you to TT text over existing text.

 Thanks for your post, we just updated the document.