Then there is no need to change the DB, but openHAB should be fixed.
I am pretty sure external temperature used to work. People reported it works in openHAB in the past.
No mention of not being able to set value 128. Also the default LCD timeout would have been reported.
You now cannot even change the name of the Thing without setting a non-default LCD timeout of 5-30.
So if you say value in the range (5-30) AND the listed options 0
(or 128 and -50 - 50 for the Temperature Offset) should work, openHAB needs to be changed to allow this. I a new to openHAB, but will have a look where this is coded.