It seems we lost three monitors basically overnight.
The main (landscape, left) displays of Pianosa, Rossa and Allegra are all broken with the same failure mode:
their backlights failed. Gautam and I confirmed that there is still an image displayed on all three, just incredibly faint. While Allegra hasn't been used much, we can narrow down that Pianosa's and Rossa's monitors must have failed within 5 or 6 hours of each other, last night.
One could say ... they turned to the dark side
Quick edit; There was a functioning Dell 24" monitor next to the iMac that we used as a replacement for Pianosa's primary display. Once the new curved display is paired with Rossa we can use its old display for Donatella or Allegra.