It's strange for sure. When DMD's outgas they usually show signs of slow "warm up" when powered on, or areas of the DMD start to flicker or get blotchy. Where your bottom 3rd of the DMD is blank and if it's a straight line cut I would start to look at the ribbon cables that carry the data from the MPU to the DMD driver and from the DMD driver to the DMD itself. Try re-seating all of the ribbon cables and see if you notice any change.
If not, hopefully you have another DMD machine you can test this DMD in just to confirm.
D