I have a similar Kewtech though mine isn't a 'plus'.
You are right, you should see the Line/Neutral voltage on the screen at that point.
From the manual:
View attachment 91970
So you are right to raise your eyebrows.
I've just noticed you said ALL upstairs sockets are like this. Are they on their own circuit or same ring as downstairs?
Have you re-proved the tester and test leads are working downstairs or elsewhere since discovering this?
I'd put the MFT down and pick up the 2 pole tester and go back to first principles, L-E and L-N tests at all sockets, maybe check wiring for circuit at CU too. Until you are seeing a normal supply at the sockets there's no point proceeding with loop tests. Logic will eventually prevail...
This is an interesting one, so do report back!