Electronic & Hydraulic Shifting (Di2, Ui2, customisations)

My 1x XT Di2 setup has stopped shifting and I'm wondering if anybody might have any insight as to why. I disconnected the wires into the display to change the stem and now the rear derailleur no longer responds to input. My setup is an SC-MT800 connected to a BT-DN110, an SW-M8050 and an RD-M8050, with no other junctions.

• Pressing the button on the display causes the screen to light up and report the correct derailleur position.
  
• Pressing and holding the button on the display until it shows 'P' does indeed cause the derailleur to shift into the largest cog.
  
• Both the Windows and the Android E-Tube apps detect the presence of all four components.
  
• Replacing the wire to the shifter with a different one does not solve the problem.
  
• Swapping the positions of the wires to the shifter and the battery at the display end does not solve the problem.
  
• Unplugging the battery for 30 seconds does not solve the problem.
  
• The battery is fully charged.
  
  

What am I missing? By process of elimination, it seems like the shifter is faulty. But how fragile are these things, and how would I have broken it by disconnecting wires and replacing a stem?