Thunderbolt Display Issues

I have a new macmini server with a thunderbolt display attached

At first there were no apparent Issues Then the display started to shut down & could only be restarted by disconnecting the Power cable

All updates are up to date including firmware

Now it goes to sleep when it should but wakes to a black screen With sound, keyboard & mouse input apparently working from the sounds But no DISPLAY

Only solution is pull the power plug Tried reconnecting the Thunderbolt cable but it has no effect.

If i setup hot corners with Put display to sleep that seems to work ok & it resumes as expected.

This happens repeatedly on some days but not allways every day