This is literally evidence of stuff being designed to fail. An extra diode costs less than a cent at production scale. This was a manufacturing choice, not an error.
I ended up replacing the mainboard with a part from no apparent manufacturer with new features (the blue LEDs dim after inactivity so as not to illuminate the whole room at night) and no connection for the thermistor. Works like a charm. It feels very much like the original manufacturer wanted the board to fail and be replaced, while some random Chinese circuit-board maker sold me a better quality board.
What would help is not randomly planning for some of the segments to fail (they are multiplexed with other things, you'd have to put more diodes), but to just get slightly better/less cheap LED display
Only "choice" made here was sorting by price when buying components for the cheap device.
As if I needed another reason to detest the eye-searing blue LEDs that have infested every device.
Theres further issues with everything coming out of china and a brand slapped on it. No one is left to take responsibility on the engineering front. This feedback I doubt will get to the correct people at best buy, let alone going back to the microwave manufacturer. And then there's the question of if they care, as they aren't a customer facing brand.
LEDs are diodes (Light emitting diode). Certainly this was a cost saving measure, but it's not a bad assumption that the LED wouldn't allow reverse current flow.
People usually respond to this by saying that it would be absurd to suggest the company did this for its own benefit, when anyone who engineers knows these are often caused by revising design to minimize costs... and increase profits.