You would have thought that, if they've got LED technology in there, they'd also put in proper diagnostics and remote monitoring so that any failures would be spotted and fixed almost immediately.

Unfortunately, this does not seem to be the case. :-(
I have also noticed that a lot of the new LED traffic lights and road signs are soon showing failed LEDs (or groups of LEDs) - and that they remain in that condition for extended periods.

I think the trouble is probably that these things are being sold on the basis, "It's solid-state - what can go wrong?!"

Now, we all know that incandescent lamps have many inherent drawbacks, but they are well-established technology, the issues are well-understood, and the authorities know the appropriate maintenance & repair strategies.

High-power LEDs, on the other hand, are still "new technology" and don't have the benefit of all that accumulated field experience.

That's my theory, anyhow...