why does my Thunderbolt display not wake from sleep
I have managed to isolate the problem. 

It seems every time a mac connected to a thunderbolt display enters automatic sleep mode, it somehow screws up the PRAM and prevents it from operating normally. 

There is a work around. 


Set "Display Sleep" to "Never" from "Energy Saver" menu in "System Preferences".
Turn off "Automatically adjust brightness" from "Display Preferences" (Note: The settings will appear to reset and your brightness setting from keyboard will stop working"
Shutdown the mac. 
Perform a PRAM reset by simultaneously pressing CMD+Option+P+R when booting up, keep keys pressed until you hear a second chime.
Power cycle your Thunderbolt Display by remove plug from socket, waiting 10seconds then plugging back in.


Your brightness keyboard button should be working now, and if you select "Sleep" from shutdown menu, the display will wake up upon key press normally.

You can also restart the station and it should perform normally.

SYSTEM
Mac Mini Mid 2011 - High Sierra 
Thunderbolt Display 27inch