Thanks, Shakalnokturn.
I have solved it ! : Small compression spring hidden inside the red switch contact carrier was simply absent! It is essential, to hold the contact pressed lightly against the switch tracks. It's absence was inferred by beeper test.
Snapping open the contact carrier was awkward to say the least. I managed to find a spring to fit, and bingo!
Unfortunately my new spring is bit harder to compress, which lifts the carrier arm an extra 0.5mm, and presses a little harder on the tracks. Finding an exact spring is a wild chase, and I would rather not try disassembing the all-plastic carrier again.
The back of the board has a large plastic shroud which also carries the display and backlight. To gain access to the back of the switch, this shroud has to come off, but won't, without desoldering at least four awkwardly located pins at the back of the lcd.
This makes cleaning impossible. Dunking the whole board in IPA is a way, but what about the pots and trimmers?
I checked DCV calibration against an U1242 calibrated by Keysight. Very good! All other functions seem to work as intended.
I need a user manual, Rishabh's support is useless. No download available anywhere.
Thank you again for help.
Edit: I got the exact spring from another Rishabh mule, perfect. R support sent me a scanned user manual, nice.